研究者データベース
| 岡本 崇(オカモト タカシ) |
| 理学研究院 物理学部門 量子物理学分野 |
| 教授 |
Last Updated :2023/12/05
基本情報
所属
職名
学位
ホームページURL
科研費研究者番号
- 50541893
J-Global ID
研究キーワード
研究分野
担当教育組織
職歴
- 2022年03月 - 現在 北海道大学 理学研究院 教授
- 2017年07月 - 2022年02月 北海道大学 理学研究院 講師
- 2013年03月 - 2017年06月 北海道大学 理学研究院 助教
- 2012年04月 - 2013年03月 筑波大学 数理物質系 准教授
- 2011年04月 - 2012年03月 - 筑波大学 数理物質科学研究科・准教授
- 2010年12月 - 2011年03月 筑波大学 計算科学研究センター・主任研究員
- 2010年 - 2011年 Senior Researcher at Center for Computational Sciences, University of Tsukuba
- 2011年 - Associate Professor at Graduate School of Pure and Applied Sciences, University of Tsukuba
- 2008年12月 - 2010年11月 筑波大学 数理物質科学研究科・助教
- 2008年 - 2010年 Assistant Professor at Graduate School of Pure and Applied Sciences, University of Tsukuba
- 2007年 - 2008年 英ダーラム大学物理学科・博士研究員
- 2007年 - 2008年 Postdoctoral Research Associate at Department of Physics, University of Durham
- 2004年 - 2007年 国立天文台理論研究部・日本学術振興会特別研究員 (PD)
- 2004年 - 2007年 JSPS Postdoctoral Research Fellow at Division of Theoretical Astronomy, National Astronomical Observatory Japan
- 2001年 - 2004年 英ダーラム大学物理学科・博士研究員
- 2001年 - 2004年 Postdoctoral Research Associate at Department of Physics, Durham University
- 2001年 京都大学理学研究科・研修員
- 2001年 京都大学基礎物理学研究所・湯川奨学研究員/非常勤講師
- 2001年 Research Associate at Graduate School of Science, Kyoto University
- 2001年 Yukawa Memorial Fellow at Yukawa Institute for Theoretical Physics, Kyoto University
学歴
所属学協会
研究活動情報
論文
- Yoichi Tamura, Tom J. L. C. Bakx, Akio K. Inoue, Takuya Hashimoto, Tsuyoshi Tokuoka, Chihiro Imamura, Bunyo Hatsukade, Minju M. Lee, Kana Moriwaki, Takashi Okamoto, Kazuaki Ota, Hideki Umehata, Naoki Yoshida, Erik Zackrisson, Masato Hagimoto, Hiroshi Matsuo, Ikkoh Shimizu, Yuma Sugahara, Tsutomu T. TakeuchiThe Astrophysical Journal 2023年07月01日 [査読有り]
- Nirmit Sakre, Asao Habe, Alex R Pettitt, Takashi Okamoto, Rei Enokiya, Yasuo Fukui, Takashi HosokawaMonthly Notices of the Royal Astronomical Society 2023年05月11日 [査読有り]
- Yi W. Ren, Yoshinobu Fudamoto, Akio K. Inoue, Yuma Sugahara, Tsuyoshi Tokuoka, Yoichi Tamura, Hiroshi Matsuo, Kotaro Kohno, Hideki Umehata, Takuya Hashimoto, Rychard J. Bouwens, Renske Smit, Nobunari Kashikawa, Takashi Okamoto, Takatoshi Shibuya, Ikkoh ShimizuThe Astrophysical Journal 945 69 2023年03月01日 [査読有り]
- Morokuma-Matsui, Kana, Bekki, Kenji, Wang, Jing, Serra, Paolo, Koyama, Yusei, Morokuma, Tomoki, Egusa, Fumi, For, Bi-Qing, Nakanishi, Kouichiro, Koribalski, Bärbel S., Okamoto, Takashi, Kodama, Tadayuki, Lee, Bumhyun, Maccagni, Filippo M., Miura, Rie E., Espada, Daniel, Takeuchi, Tsutomu T., Yang, Dong, Lee, Minju M., Ueda, Masaki, Matsushita, KyokoThe Astrophysical Journal Supplement Series 2022年12月 [査読有り]
We conduct a 12C16O(J = 1-0) (hereafter CO) mapping survey of 64 galaxies in the Fornax cluster using the Atacama Large Millimeter/submillimeter Array Morita array in cycle 5. CO emission is detected from 23 out of the 64 galaxies. Our sample includes dwarf, spiral, and elliptical galaxies with stellar masses of M star ~ 106.3-11.6 M ⊙. The achieved beam size and sensitivity are 15″ × 8″ and ~12 mJy beam-1 at the velocity resolution of ~10 km s-1, respectively. We study the cold gas (molecular and atomic gas) properties of 38 subsamples with M star > 109 M ⊙ combined with literature H I data. We find that (1) the low star formation (SF) activity in the Fornax galaxies is caused by the decrease in the cold gas mass fraction with respect to stellar mass (hereafter, gas fraction) rather than the decrease of the SF efficiency from the cold gas; (2) the atomic gas fraction is more heavily reduced than the molecular gas fraction of such galaxies with low SF activity. A comparison between the cold gas properties of the Fornax galaxies and their environmental properties suggests that the atomic gas is stripped tidally and by the ram pressure, which leads to the molecular gas depletion with an aid of the strangulation and consequently SF quenching. Preprocesses in the group environment would also play a role in reducing cold gas reservoirs in some Fornax galaxies.... - Yutaka Hirai, Timothy C Beers, Masashi Chiba, Wako Aoki, Derek Shank, Takayuki R Saitoh, Takashi Okamoto, Junichiro MakinoMonthly Notices of the Royal Astronomical Society 2022年11月14日 [査読有り]
- Masato Shirasaki, Takashi Okamoto, Shin’ichiro AndoMonthly Notices of the Royal Astronomical Society 2022年09月23日 [査読有り]
- Nashimoto, Masashi, Tanaka, Masayuki, Chiba, Masashi, Hayashi, Kohei, Komiyama, Yutaka, Okamoto, TakashiThe Astrophysical Journal 2022年09月 [査読有り]
We present a new observation of satellite galaxies around seven Milky Way (MW)-like galaxies located outside of the Local Group (LG) using Subaru/Hyper Suprime-Cam imaging data to statistically address the missing satellite problem. We select satellite galaxy candidates using magnitude, surface brightness, Sérsic index, axial ratio, FWHM, and surface brightness fluctuation cuts, followed by visual screening of false positives such as optical ghosts of bright stars. We identify 51 secure dwarf satellite galaxies within the virial radius of nine host galaxies, two of which are drawn from the pilot observation presented in Paper I. We find that the average luminosity function of the satellite galaxies is consistent with that of the MW satellites, although the luminosity function of each host galaxy varies significantly. We observe an indication that more massive hosts tend to have a larger number of satellites. Physical properties of the satellites such as the size-luminosity relation are also consistent with the MW satellites. However, the spatial distribution is different; we find that the satellite galaxies outside of the LG show no sign of concentration or alignment, while that of the MW satellites is more concentrated around the host and exhibits a significant alignment. As we focus on relatively massive satellites with M V < -10, we do not expect that the observational incompleteness can be responsible here. This trend might represent a peculiarity of the MW satellites, and further work is needed to understand its origin.... - Shigeki Inoue, Xiaotian Si, Takashi Okamoto, Moka NishigakiMonthly Notices of the Royal Astronomical Society 2022年08月09日 [査読有り]
- Elizabeth J Iles, Alex R Pettitt, Takashi OkamotoMonthly Notices of the Royal Astronomical Society 2022年01月13日 [査読有り]
- Kana Morokuma-Matsui, Tadayuki Kodama, Tomoki Morokuma, Kouichiro Nakanishi, Yusei Koyama, Takuji Yamashita, Shuhei Koyama, Takashi OkamotoThe Astrophysical Journal 2021年06月01日 [査読有り]
- Testing the effect of resolution on gravitational fragmentation with Lagrangian hydrodynamic schemesYasuyoshi Yamamoto, Takashi Okamoto, Takayuki R SaitohMonthly Notices of the Royal Astronomical Society 504 3 3986 - 3995 2021年05月11日 [査読有り]
ABSTRACT To study the resolution required for simulating gravitational fragmentation with newly developed Lagrangian hydrodynamic schemes, meshless finite-volume method (MFV) and meshless finite-mass method, we have performed a number of simulations of the Jeans test and compared the results with both the expected analytical solution and results from the more standard Lagrangian approach: smoothed particle hydrodynamics (SPH). We find that the different schemes converge to the analytical solution when the diameter of a fluid element is smaller than a quarter of the Jeans wavelength, λJ. Among the three schemes, SPH/MFV shows the fastest/slowest convergence to the analytical solution. Unlike the well-known behaviour of Eulerian schemes, none of the Lagrangian schemes investigated displays artificial fragmentation when the perturbation wavelength, λ, is shorter than λJ, even at low numerical resolution. For larger wavelengths (λ > λJ), the growth of the perturbation is delayed when it is not well resolved. Furthermore, with poor resolution, the fragmentation seen with the MFV scheme proceeds very differently compared to the converged solution. All these results suggest that, when unresolved, the ratio of the magnitude of hydrodynamic force to that of self-gravity at the sub-resolution scale is the largest/smallest in MFV/SPH, the reasons for which we have discussed in detail. These tests are repeated to investigate the effect of kernels of higher order than the fiducial cubic spline. Our results indicate that the standard deviation of the kernel is a more appropriate definition of the ‘size’ of a fluid element than its compact support radius. - C. Binggeli, A. K. Inoue, T. Hashimoto, M. C. Toribio, E. Zackrisson, S. Ramstedt, K. Mawatari, Y. Harikane, H. Matsuo, T. Okamoto, K. Ota, I. Shimizu, Y. Tamura, Y. Taniguchi, H. UmehataAstronomy & Astrophysics 646 A26 - A26 2021年02月 [査読有り]
Context. Characterizing the galaxy population in the early Universe holds the key to understanding the evolution of these objects and the role they played in cosmic reionization. However, there have been very few observations at the very highest redshifts to date.Aims. In order to shed light on the properties of galaxies in the high-redshift Universe and their interstellar media, we observe the Lyman-α emitting galaxyz7_GSD_3811 atz = 7.664 with bands 6 and 8 at the Atacama Large Millimeter/submillimeter Array (ALMA).Methods. We target the far-infrared [OIII ] 88μ m and [CII ] 158μ m emission lines and dust continuum in the star-forming galaxy z7_GSD_3811 with ALMA. We combine these measurements with earlier observations in the rest-frame ultraviolet (UV) in order to characterize the object and compare the results to those of earlier studies that observed [OIII ] and [CII ] emission in high-redshift galaxies.Results. The [OIII ] 88μ m and [CII ] 158μ m emission lines are undetected at the position of z7_GSD_3811, with 3σ upper limits of 1.6 × 108L ⊙ and 4.0 × 107L ⊙, respectively. We do not detect any dust continuum in band 6 nor band 8. The measured rms in the band 8 and band 6 continua are 26 and 9.9μ Jy beam−1, respectively. Similar to several other high-redshift galaxies, z7_GSD_3811 exhibits low [CII ] emission for its star formation rate compared to local galaxies. Furthermore, our upper limit on the [OIII ] line luminosity is lower than the previously observed [OIII ] lines in high-redshift galaxies with similar UV luminosities. Our ALMA band 6 and 8 dust continuum observations imply that z7_GSD_3811 likely has a low dust content, and our non-detections of the [OIII ] and [CII ] lines could indicate that z7_GSD_3811 has a low metallicity (Z ≲ 0.1Z ⊙). - Sakre, Nirmit, Habe, Asao, Pettitt, Alex R., Okamoto, TakashiPublications of the Astronomical Society of Japan 2021年01月 [査読有り]
We study the effect of magnetic field on massive dense core formation in colliding unequal molecular clouds by performing magnetohydrodynamic simulations with sub-parsec resolution (0.015 pc) that can resolve the molecular cores. Initial clouds with the typical gas density of the molecular clouds are immersed in various uniform magnetic fields. The turbulent magnetic fields in the clouds consistent with the observation by Crutcher et al. (2010, ApJ, 725, 466) are generated by the internal turbulent gas motion before the collision, if the uniform magnetic field strength is 4.0 μG. The collision speed of 10 km s-1 is adopted, which is much larger than the sound speeds and the Alfvén speeds of the clouds. We identify gas clumps with gas densities greater than 5 × 10-20 g cm-3 as the dense cores and trace them throughout the simulations to investigate their mass evolution and gravitational boundness. We show that a greater number of massive, gravitationally bound cores are formed in the strong magnetic field (4.0 μG) models than the weak magnetic field (0.1 μG) models. This is partly because the strong magnetic field suppresses the spatial shifts of the shocked layer that should be caused by the nonlinear thin shell instability. The spatial shifts promote the formation of low-mass dense cores in the weak magnetic field models. The strong magnetic fields also support low-mass dense cores against gravitational collapse. We show that the numbers of massive, gravitationally bound cores formed in the strong magnetic field models are much larger than in the isolated, non-colliding cloud models, which are simulated for comparison. We discuss the implications of our numerical results on massive star formation.... - Taira Oogi, Hikari Shirakata, Masahiro Nagashima, Takahiro Nishimichi, Toshihiro Kawaguchi, Takashi Okamoto, Tomoaki Ishiyama, Motohiro EnokiMonthly Notices of the Royal Astronomical Society 497 1 1 - 18 2020年09月01日 [査読有り]
The spatial clustering of active galactic nuclei (AGNs) is considered to be one of the important diagnostics for the understanding of the underlying processes behind their activities complementary to measurements of the luminosity function (LF). We analyse the AGN clustering from a recent semi-analytic model performed on a large cosmological N-body simulation covering a cubic gigaparsec comoving volume. We have introduced a new time-scale of gas accretion on to the supermassive black holes to account for the loss of the angular momentum on small scales, which is required to match the faint end of the observed X-ray LF. The large simulation box allows us accurate determination of the autocorrelation function of the AGNs. The model prediction indicates that this time-scale plays a significant role in allowing massive haloes to host relatively faint population of AGNs, leading to a higher bias factor for those AGNs. The model predictions are in agreement with observations of X-ray selected AGNs in the luminosity range 10(41.5) erg s(-1) <= L-2 (10keV) <= 10(44.5) erg s(-1), with the typical host halo mass of 10(12.5-13.5)h(-1) M-circle dot at z less than or similar to 1. This result shows that the observational clustering measurements impose an independent constraint on the accretion time-scale complementary to the LF measurements. Moreover, we find that not only the effective halo mass corresponding to the overall bias factor, but the extended shape of the predicted AGN correlation function shows remarkable agreement with those from observations. Further observational efforts towards the low-luminosity end at z similar to 1 would give us stronger constraints on the triggering mechanisms of AGN activities through their clustering. - Sakre, Nirmit, Habe, Asao, Pettitt, Alex R., Okamoto, TakashiPublications of the Astronomical Society of Japan 2020年07月 [査読有り]
We study the effect of magnetic field on massive dense core formation in colliding unequal molecular clouds by performing magnetohydrodynamic simulations with sub-parsec resolution (0.015 pc) that can resolve the molecular cores. Initial clouds with the typical gas density of the molecular clouds are immersed in various uniform magnetic fields. The turbulent magnetic fields in the clouds consistent with the observation by Crutcher et al. (2010, ApJ, 725, 466) are generated by the internal turbulent gas motion before the collision, if the uniform magnetic field strength is 4.0 μG. The collision speed of 10 km s-1 is adopted, which is much larger than the sound speeds and the Alfvén speeds of the clouds. We identify gas clumps with gas densities greater than 5 × 10-20 g cm-3 as the dense cores and trace them throughout the simulations to investigate their mass evolution and gravitational boundness. We show that a greater number of massive, gravitationally bound cores are formed in the strong magnetic field (4.0 μG) models than the weak magnetic field (0.1 μG) models. This is partly because the strong magnetic field suppresses the spatial shifts of the shocked layer that should be caused by the nonlinear thin shell instability. The spatial shifts promote the formation of low-mass dense cores in the weak magnetic field models. The strong magnetic fields also support low-mass dense cores against gravitational collapse. We show that the numbers of massive, gravitationally bound cores formed in the strong magnetic field models are much larger than in the isolated, non-colliding cloud models, which are simulated for comparison. We discuss the implications of our numerical results on massive star formation.... - Shirakata, Hikari, Kawaguchi, Toshihiro, Okamoto, Takashi, Nagashima, Masahiro, Oogi, TairaThe Astrophysical Journal 898 1 2020年07月 [査読有り]
We show the significance of the super-Eddington accretion for the cosmic growth of supermassive black holes (SMBHs) with a semianalytical model for galaxy and black hole evolution. The model explains various observed properties of galaxies and active galactic nuclei at a wide redshift range. By tracing the growth history of individual SMBHs, we find that the fraction of the SMBH mass acquired during the super-Eddington accretion phases to the total SMBH mass becomes larger for less massive black holes and at higher redshift. Even at z ∼ 0, SMBHs with $\gt {10}^{9},{M}_{\odot }$ have acquired more than 50% of their mass by super-Eddington accretions, which is apparently inconsistent with the classical Sołtan argument. However, the mass-weighted radiation efficiency of SMBHs with $\gt {10}^{8},{M}_{\odot }$ obtained with our model, is about 0.08 at z ∼ 0, which is consistent with Sołtan's argument within the observational uncertainties. We, therefore, conclude that Sołtan's argument cannot reject the possibility that SMBHs are grown mainly by super-Eddington accretions.... - Bakx, Tom J. L. C., Tamura, Yoichi, Hashimoto, Takuya, Inoue, Akio K., Lee, Minju M., Mawatari, Ken, Ota, Kazuaki, Umehata, Hideki, Zackrisson, Erik, Hatsukade, Bunyo, Kohno, Kotaro, Matsuda, Yuichi, Matsuo, Hiroshi, Okamoto, Takashi, Shibuya, Takatoshi, Shimizu, Ikkoh, Taniguchi, Yoshiaki, Yoshida, NaokiMonthly Notices of the Royal Astronomical Society 2020年04月 [査読有り]
We report on the detection of the [C II] 157.7 μm emission from the Lyman break galaxy (LBG) MACS0416_Y1 at z = 8.3113, by using the Atacama Large Millimeter/submillimeter Array (ALMA). The luminosity ratio of [O III] 88 μm (from previous campaigns) to [C II] is 9.3 ± 2.6, indicative of hard interstellar radiation fields and/or a low covering fraction of photodissociation regions. The emission of [C II] is cospatial to the 850 μm dust emission (90 μm rest frame, from previous campaigns), however the peak [C II] emission does not agree with the peak [O III] emission, suggesting that the lines originate from different conditions in the interstellar medium. We fail to detect continuum emission at 1.5 mm (160 μm rest frame) down to 18 μJy (3σ). This non-detection places a strong limits on the dust spectrum, considering the 137 ± 26 μJy continuum emission at 850 μm. This suggests an unusually warm dust component (T > 80 K, 90 per cent confidence limit), and/or a steep dust-emissivity index (βdust > 2), compared to galaxy-wide dust emission found at lower redshifts (typically T ∼ 30-50 K, βdust ∼ 1-2). If such temperatures are common, this would reduce the required dust mass and relax the dust production problem at the highest redshifts. We therefore warn against the use of only single-wavelength information to derive physical properties, recommend a more thorough examination of dust temperatures in the early Universe, and stress the need for instrumentation that probes the peak of warm dust in the Epoch of Reionization.... - Tom J. L. C. Bakx, Yoichi Tamura, Takuya Hashimoto, Akio K. Inoue, Minju M. Lee, Ken Mawatari, Kazuaki Ota, Hideki Umehata, Erik Zackrisson, Bunyo Hatsukade, Kotaro Kohno, Yuichi Matsuda, Hiroshi Matsuo, Takashi Okamoto, Takatoshi Shibuya, Ikkoh Shimizu, Yoshiaki Taniguchi, Naoki YoshidaMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 493 3 4294 - 4307 2020年04月 [査読有り][通常論文]
We report on the detection of the [C II] 157.7 mu m emission from the Lyman break galaxy (LBG) MACS0416_Y1 at z = 8.3113, by using the Atacama Large Millimeter/submillimeter Array (ALMA). The luminosity ratio of [O III] 88 mu m (from previous campaigns) to [CII] is 9.3 +/- 2.6, indicative of hard interstellar radiation fields and/or a low covering fraction of photodissociation regions. The emission of [C II] is cospatial to the 850 mu m dust emission (90 mu m rest frame, from previous campaigns), however the peak [C II] emission does not agree with the peak [O III] emission, suggesting that the lines originate from different conditions in the interstellar medium. We fail to detect continuum emission at 1.5 mm (160 mu m rest frame) down to 18 mu Jy (3 sigma). This non-detection places a strong limits on the dust spectrum, considering the 137 +/- 26 mu Jy continuum emission at 850 mu m. This suggests an unusually warm dust component (T > 80 K, 90 per cent confidence limit), and/or a steep dust-emissivity index (beta(dust) > 2), compared to galaxy-wide dust emission found at lower redshifts (typically T similar to 30-50 K, beta(dust) similar to 1-2). If such temperatures are common, thiswould reduce the required dust mass and relax the dust production problem at the highest redshifts. We therefore warn against the use of only single-wavelength information to derive physical properties, recommend a more thorough examination of dust temperatures in the early Universe, and stress the need for instrumentation that probes the peak of warm dust in the Epoch of Reionization. - Y. Tamura, K. Mawatari, T. Hashimoto, A. K. Inoue, E. Zackrissonm, L. Christensen, C. Binggeli, Y. Matsuda, H. Matsuo, T. T. Takeuchi, R. S. Asano, K. Sunaga, I. Shimizu, T. Okamoto, N. Yoshida, M. Lee, T. Shibuya, Y. Taniguchi, H. Umehata, B. Hatsukade, K. Kohno, K. OtaProceedings of the International Astronomical Union 15 S341 211 - 215 2019年11月 [査読有り]
Abstract We present ALMA detection of the [Oiii ] 88μ m line and 850μ m dust continuum emission in aY -dropout Lyman break galaxy, MACS0416_Y1. The [Oiii ] detection confirms the object with a spectroscopic redshift to bez = 8.3118±0.0003. The 850μ m continuum intensity (0.14 mJy) implies a large dust mass on the order of 4×106M ⊙. The ultraviolet-to-far infrared spectral energy distribution modeling, where the [Oiii ] emissivity model is incorporated, suggests the presence of a young (τage ≍ 4 Myr), star-forming (SFR ≍ 60M ⊙yr−1), and moderately metal-polluted (Z ≍ 0.2Z ⊙) stellar component with a stellar mass of 3 × 108M ⊙. An analytic dust mass evolution model with a single episode of star formation does not reproduce the metallicity and dust mass in ≍ 4 Myr, suggesting an underlying evolved stellar component as the origin of the dust mass. - Hashimoto, Takuya, Inoue, Akio K., Mawatari, Ken, Tamura, Yoichi, Matsuo, Hiroshi, Furusawa, Hisanori, Harikane, Yuichi, Shibuya, Takatoshi, Knudsen, Kirsten K., Kohno, Kotaro, Ono, Yoshiaki, Zackrisson, Erik, Okamoto, Takashi, Kashikawa, Nobunari, Oesch, Pascal A., Ouchi, Masami, Ota, Kazuaki, Shimizu, Ikkoh, Taniguchi, Yoshiaki, Umehata, Hideki, Watson, DarachPublications of the Astronomical Society of Japan 71 4 2019年08月 [査読有り][通常論文]
We present new ALMA observations and physical properties of a Lyman break galaxy at z = 7.15. Our target, B14-65666, has a bright ultra-violet (UV) absolute magnitude, MUV ≈ -22.4, and has been spectroscopically identified in Lyα with a small rest-frame equivalent width of ≈4 Å. A previous Hubble Space TElescope (HST) image has shown that the target is composed of two spatially separated clumps in the rest-frame UV. With ALMA, we have newly detected spatially resolved [O III] 88 μm, [C II] 158 μm, and their underlying dust continuum emission. In the whole system of B14-65666, the [O III] and [C II] lines have consistent redshifts of 7.1520 ± 0.0003, and the [O III] luminosity, (34.4 ± 4.1) × 108 L☉, is about three times higher than the [C II] luminosity, (11.0 ± 1.4) × 108 L☉. With our two continuum flux densities, the dust temperature is constrained to be Td ≈ 50-60 K under the assumption of a dust emissivity index of βd = 2.0-1.5, leading to a large total infrared luminosity of LTIR ≈ 1 × 1012 L☉. Owing to our high spatial resolution data, we show that the [O III] and [C II] emission can be spatially decomposed into two clumps associated with the two rest-frame UV clumps whose spectra are kinematically separated by ≈200 km s-1. We also find these two clumps have comparable UV, infrared, [O III], and [C II] luminosities. Based on these results, we argue that B14-65666 is a starburst galaxy induced by a major merger. The merger interpretation is also supported by the large specific star formation rate (defined as the star formation rate per unit stellar mass), sSFR = 260^{+119}_{-57}Gyr-1, inferred from our SED fitting. Probably, a strong UV radiation field caused by intense star formation contributes to its high dust temperature and the [O III]-to-[C II] luminosity ratio.... - Shirakata, Hikari, Kawaguchi, Toshihiro, Oogi, Taira, Okamoto, Takashi, Nagashima, MasahiroMonthly Notices of the Royal Astronomical Society 487 1 409 - 419 2019年07月 [査読有り][通常論文]
We show the Eddington ratio distributions of supermassive black holes at a wide redshift range (0 < z < 8) obtained with a semi-analytic model of galaxy formation. The distribution is broadly consistent with observational estimates at low redshift. We find that the growth rate of black holes at higher redshift is more likely to exceed the Eddington limit because the typical gas fraction of the host galaxies is higher at higher redshift. We also find that the super-Eddington growth is more common for less massive supermassive black holes, supporting an idea that supermassive black holes have been formed via super-Eddington accretion. These results indicate the `slowing down' of cosmic growth of supermassive black holes: the growth of supermassive black holes with a higher Eddington ratio peaks at higher redshift. We also show the effect of the sample selection on the shape of the Eddington ratio distribution functions and find that shallower observations will miss active galactic nuclei with not only the smaller but also higher Eddington ratios.... - Tamura, Yoichi, Mawatari, Ken, Hashimoto, Takuya, Inoue, Akio K., Zackrisson, Erik, Christensen, Lise, Binggeli, Christian, Matsuda, Yuichi, Matsuo, Hiroshi, Takeuchi, Tsutomu T., Asano, Ryosuke S., Sunaga, Kaho, Shimizu, Ikkoh, Okamoto, Takashi, Yoshida, Naoki, Lee, Minju M., Shibuya, Takatoshi, Taniguchi, Yoshiaki, Umehata, Hideki, Hatsukade, Bunyo, Kohno, Kotaro, Ota, KazuakiThe Astrophysical Journal 874 1 2019年03月 [査読有り][通常論文]
We present the Atacama Large Millimeter/submillimeter Array detection of the [O III] 88 μm line and rest-frame 90 μm dust continuum emission in a Y-dropout Lyman break galaxy (LBG), MACS0416_Y1 lying behind the Frontier Field cluster MACS J0416.1-2403. This [O III] detection confirms the LBG with a spectroscopic redshift of z = 8.3118 ± 0.0003, making this object one of the farthest galaxies ever identified spectroscopically. The observed 850 μm flux density of 137 ± 26 μJy corresponds to a de-lensed total infrared (IR) luminosity of {L}IR}=(1.7+/- 0.3)× {10}11 {L}☉ if assuming a dust temperature of T dust = 50 K and an emissivity index of β = 1.5, yielding a large dust mass of 4× {10}6{M}☉ . The ultraviolet-to-far-IR spectral energy distribution modeling where the [O III] emissivity model is incorporated suggests the presence of a young (τ age ≈ 4 Myr), star-forming ({SFR}≈ 60 {M}☉ yr-1), moderately metal-polluted (Z ≈ 0.2Z ☉) stellar component with a mass of M star = 3 × 108 M ☉. An analytic dust mass evolution model with a single episode of star formation does not reproduce the metallicity and dust mass in τ age ≈ 4 Myr, suggesting a pre-existing evolved stellar component with M star ∼ 3 × 109 M ☉ and τ age ∼ 0.3 Gyr as the origin of the dust mass.... - Hikari Shirakata, Takashi Okamoto, Toshihiro Kawaguchi, Masahiro Nagashima, Tomoaki Ishiyama, Ryu Makiya, Masakazu A, R. Kobayashi, Motohiro Enoki, Taira Oogi, Katsuya OkoshiMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 482 4 4846 - 4873 2019年02月 [査読有り][通常論文]
We present the latest results of a semi-analytic model of galaxy formation,
"New Numerical Galaxy Catalogue", which is combined with large cosmological
N-body simulations. This model can reproduce statistical properties of galaxies
at z < 6.0. We focus on the properties of active galactic nuclei (AGNs) and
supermassive black holes, especially on the accretion timescale onto black
holes. We find that the number density of AGNs at z < 1.5 and at hard X-ray
luminosity 10^{ 44 }< erg/s is underestimated compared with recent
observational estimates when we assume the exponentially decreasing accretion
rate and the accretion timescale which is proportional to the dynamical time of
the host halo or the bulge, as is often assumed in semi-analytic models. We
show that to solve this discrepancy, the accretion timescale of such less
luminous AGNs instead should be a function of the black hole mass and the
accreted gas mass. This timescale can be obtained from a phenomenological
modelling of the gas angular momentum loss in the circumnuclear torus and/or
the accretion disc. Such models predict a longer accretion timescale for less
luminous AGNs at z < 1.0 than bright QSOs whose accretion timescale would be
10^{ 7-8 } yr. With this newly introduced accretion timescale, our model can
explain the observed luminosity functions of AGNs at z < 6.0. - Tanaka, Masayuki, Chiba, Masashi, Hayashi, Kohei, Komiyama, Yutaka, Okamoto, Takashi, Cooper, Andrew P., Okamoto, Sakurako, Spitler, LeeThe Astrophysical Journal 865 2 id125 - 12 2018年10月 [査読有り][通常論文]
We present results from a pilot observation of nearby (∼20 Mpc) galaxies with masses similar to that of the Milky Way (MW) to address the missing satellite problem. This is the first paper from an ongoing project to address the problem with a statistical sample of galaxies outside of the Local Group (LG) without employing an assumption that the LG is a typical halo in the universe. Thanks to the close distances of our targets, dwarf galaxies around them can be identified as extended, diffuse galaxies. By applying a surface brightness cut together with a careful visual screening to remove artifacts and background contamination, we construct a sample of dwarf galaxies. The luminosity function (LF) of one of the targets is broadly consistent with that of the MW, but the other has a more abundant dwarf population. Numerical simulations by Okamoto seem to overpredict the number of dwarfs on average, while more recent predictions from Copernicus Complexio are in better agreement. In both observations and simulations, there is a large diversity in the LFs, demonstrating the importance of addressing the missing satellite problem with a statistically representative sample. We also characterize the projected spatial distributions of the satellites and do not observe strong evidence for alignments around the central galaxies. Based on this successful pilot observation, we are carrying out further observations to increase the sample of nearby galaxies, which we plan to report in our future paper.... - Hashimoto, Takuya, Laporte, Nicolas, Mawatari, Ken, Ellis, Richard S., Inoue, Akio K., Zackrisson, Erik, Roberts-Borsani, Guido, Zheng, Wei, Tamura, Yoichi, Bauer, Franz E., Fletcher, Thomas, Harikane, Yuichi, Hatsukade, Bunyo, Hayatsu, Natsuki H., Matsuda, Yuichi, Matsuo, Hiroshi, Okamoto, Takashi, Ouchi, Masami, Pelló, Roser, Rydberg, Claes-Erik, Shimizu, Ikkoh, Taniguchi, Yoshiaki, Umehata, Hideki, Yoshida, NaokiNature 557 7705 392 - + 2018年05月 [査読有り][通常論文]
A fundamental quest of modern astronomy is to locate the earliest galaxies and study how they influenced the intergalactic medium a few hundred million years after the Big Bang1-3. The abundance of star-forming galaxies is known to decline4,5 from redshifts of about 6 to 10, but a key question is the extent of star formation at even earlier times, corresponding to the period when the first galaxies might have emerged. Here we report spectroscopic observations of MACS1149-JD16, a gravitationally lensed galaxy observed when the Universe was less than four per cent of its present age. We detect an emission line of doubly ionized oxygen at a redshift of 9.1096 ± 0.0006, with an uncertainty of one standard deviation. This precisely determined redshift indicates that the red rest-frame optical colour arises from a dominant stellar component that formed about 250 million years after the Big Bang, corresponding to a redshift of about 15. Our results indicate that it may be possible to detect such early episodes of star formation in similar galaxies with future telescopes.... - Nguyen, Ngan K., Pettitt, Alex R., Tasker, Elizabeth J., Okamoto, TakashiMonthly Notices of the Royal Astronomical Society 475 1 27 - 42 2018年03月 [査読有り][通常論文]
We explore the effect of different galactic disc environments on the properties of star-forming clouds through variations in the background potential in a set of isolated galaxy simulations. Rising, falling, and flat rotation curves expected in halo-dominated, disc-dominated, and Milky Way-like galaxies were considered, with and without an additional two-arm spiral potential. The evolution of each disc displayed notable variations that are attributed to different regimes of stability, determined by shear and gravitational collapse. The properties of a typical cloud were largely unaffected by the changes in rotation curve, but the production of small and large cloud associations was strongly dependent on this environment. This suggests that while differing rotation curves can influence where clouds are initially formed, the average bulk properties are effectively independent of the global environment. The addition of a spiral perturbation made the greatest difference to cloud properties, successfully sweeping the gas into larger, seemingly unbound, extended structures and creating large arm-interarm contrasts.... - Ishiki, Shohei, Okamoto, Takashi, Inoue, Akio K.Monthly Notices of the Royal Astronomical Society 474 2 1935 - 1943 2018年02月 [査読有り][通常論文]
We investigate the impact of radiation pressure on spatial dust distribution inside H II regions using one-dimensional radiation hydrodynamic simulations, which include absorption and re-emission of photons by dust. In order to investigate grain-size effects as well, we introduce two additional fluid components describing large and small dust grains in the simulations. Relative velocity between dust and gas strongly depends on the drag force. We include collisional drag force and coulomb drag force. We find that, in a compact H II region, a dust cavity region is formed by radiation pressure. Resulting dust cavity sizes (∼0.2 pc) agree with observational estimates reasonably well. Since dust inside an H II region is strongly charged, relative velocity between dust and gas is mainly determined by the coulomb drag force. Strength of the coulomb drag force is about 2 order of magnitude larger than that of the collisional drag force. In addition, in a cloud of mass 105 M☉, we find that the radiation pressure changes the grain-size distribution inside H II regions. Since large (0.1 μm) dust grains are accelerated more efficiently than small (0.01 μm) grains, the large-to-small grain mass ratio becomes smaller by an order of magnitude compared with the initial one. Resulting dust-size distributions depend on the luminosity of the radiation source. The large and small grain segregation becomes weaker when we assume stronger radiation source, since dust grain charges become larger under stronger radiation and hence coulomb drag force becomes stronger.... - Oogi, Taira, Enoki, Motohiro, Ishiyama, Tomoaki, Kobayashi, Masakazu A. R., Makiya, Ryu, Nagashima, Masahiro, Okamoto, Takashi, Shirakata, HikariMonthly Notices of the Royal Astronomical Society 471 1 L21 - L25 2017年10月 [査読有り][通常論文]
Super-Eddington mass accretion has been suggested as an efficient mechanism to grow supermassive black holes. We investigate the imprint left by the radiative efficiency of the super-Eddington accretion process on the clustering of quasars using a new semi-analytic model of galaxy and quasar formation based on large-volume cosmological N-body simulations. Our model includes a simple model for the radiative efficiency of a quasar, which imitates the effect of photon trapping for a high mass accretion rate. We find that the model of radiative efficiency affects the relation between the quasar luminosity and the quasar host halo mass. The quasar host halo mass has only weak dependence on quasar luminosity when there is no upper limit for quasar luminosity. On the other hand, it has significant dependence on quasar luminosity when the quasar luminosity is limited by its Eddington luminosity. In the latter case, the quasar bias also depends on the quasar luminosity, and the quasar bias of bright quasars is in agreement with observations. Our results suggest that the quasar clustering studies can provide a constraint on the accretion disc model.... - Hikari Shirakata, Toshihiro Kawaguchi, Takashi Okamoto, Ryu Makiya, Tomoaki Ishiyama, Yoshiki Matsuoka, Masahiro Nagashima, Motohiro Enoki, Taira Oogi, Masakazu A. R. KobayashiFrontiers in Astronomy and Space Sciences 4 2017年09月21日 [査読有り]
- Ishiki, Shohei, Okamoto, TakashiMonthly Notices of the Royal Astronomical Society 466 1 L123 - L127 2017年03月 [査読有り][通常論文]
We have investigated the impact of photoionization and radiation pressure on a dusty star-forming cloud using one-dimensional radiation hydrodynamic simulations, which include absorption and re-emission of photons by dust. We find that, in a cloud of mass 105 M☉ and radius 17 pc, the effect of radiation pressure is negligible when star formation efficiency is 2 per cent. The importance of radiation pressure increases with increasing star formation efficiency or an increasing dust-to-gas mass ratio. The net effect of radiation feedback, however, becomes smaller with the increasing dust-to-gas mass ratio, since the absorption of ultraviolet photons by dust grains suppresses photoionization and hence photoheating.... - Okamoto, Takashi, Nagashima, Masahiro, Lacey, Cedric G., Frenk, Carlos S.Monthly Notices of the Royal Astronomical Society 464 4 4866 - 4874 2017年02月 [査読有り][通常論文]
Massive early-type galaxies have higher metallicities and higher ratios of α elements to iron than their less massive counterparts. Reproducing these correlations has long been a problem for hierarchical galaxy formation theory, both in semi-analytic models and cosmological hydrodynamic simulations. We show that a simulation in which gas cooling in massive dark haloes is quenched by radio-mode active galactic nuclei (AGNs) feedback naturally reproduces the observed trend between α/Fe and the velocity dispersion of galaxies, σ. The quenching occurs earlier for more massive galaxies. Consequently, these galaxies complete their star formation before α/Fe is diluted by the contribution from Type Ia supernovae. For galaxies more massive than ∼1011 M☉, whose α/Fe correlates positively with stellar mass, we find an inversely correlated mass-metallicity relation. This is a common problem in simulations in which star formation in massive galaxies is quenched either by quasar- or radio-mode AGN feedback. The early suppression of gas cooling in progenitors of massive galaxies prevents them from recapturing enriched gas ejected as winds. Simultaneously reproducing the [α/Fe]-σ relation and the mass-metallicity relation is, thus, difficult in the current framework of galaxy formation.... - Shohei Ishiki, Takashi OkamotoMonthly Notices of the Royal Astronomical Society: Letters 466 1 L123 - L127 2017年 [査読有り][通常論文]
We have investigated the impact of photoionization and radiation pressure on a dusty starforming cloud using one-dimensional radiation hydrodynamic simulations, which include absorption and re-emission of photons by dust. We find that, in a cloud of mass 105M⊙ and radius 17 pc, the effect of radiation pressure is negligible when star formation efficiency is 2 per cent. The importance of radiation pressure increases with increasing star formation efficiency or an increasing dust-to-gas mass ratio. The net effect of radiation feedback, however, becomes smaller with the increasing dust-to-gas mass ratio, since the absorption of ultraviolet photons by dust grains suppresses photoionization and hence photoheating. - Shirakata, Hikari, Kawaguchi, Toshihiro, Okamoto, Takashi, Makiya, Ryu, Ishiyama, Tomoaki, Matsuoka, Yoshiki, Nagashima, Masahiro, Enoki, Motohiro, Oogi, Taira, Kobayashi, Masakazu A. R.Monthly Notices of the Royal Astronomical Society 461 4 4389 - 4394 2016年10月 [査読有り][通常論文]
We explore the effect of varying the mass of a seed black hole on the resulting black hole mass-bulge mass relation at z ∼ 0, using a semi-analytic model of galaxy formation combined with large cosmological N-body simulations. We constrain our model by requiring that the observed properties of galaxies at z ∼ 0 are reproduced. In keeping with previous semi-analytic models, we place a seed black hole immediately after a galaxy forms. When the mass of the seed is set at 105 M☉, we find that the model results become inconsistent with recent observational results of the black hole mass-bulge mass relation for dwarf galaxies. In particular, the model predicts that bulges with ∼109 M☉ harbour larger black holes than observed. On the other hand, when we employ seed black holes of 103 M☉ or select their mass randomly within a 103-5 M☉ range, the resulting relation is consistent with observation estimates, including the observed dispersion. We find that, to obtain stronger constraints on the mass of seed black holes, observations of less massive bulges at z ∼ 0 are a more powerful comparison than the relations at higher redshifts.... - Shimizu, Ikkoh, Inoue, Akio K., Okamoto, Takashi, Yoshida, NaokiMonthly Notices of the Royal Astronomical Society 461 4 3563 - 3575 2016年10月 [査読有り][通常論文]
We have performed very large and high-resolution cosmological hydrodynamic simulations in order to investigate detectability of nebular lines in the rest-frame ultraviolet (UV) to optical wavelength range from galaxies at z > 7. We find that the expected line fluxes are very well correlated with the apparent UV magnitudes. The C IV 1549 Å and C III] 1909 Å lines of galaxies brighter than 26 AB magnitudes are detectable with current facilities such as the Very Large Telescope (VLT) XShooter and the Keck Multi-Object Spectrometer for Infra-Red Exploration (MOSFIRE). Metal lines such as C IV 1549 Å, C III] 1909 Å, [O II] 3727 Å and [O III] 4959/5007 Å are good targets for spectroscopic observation with the Thirty-Metre Telescope (TMT), European Extremely Large Telescope (E-ELT), Giant Magellan Telescope (GMT) and James Webb Space Telescope (JWST). We also expect Hα and Hβ lines to be detectable with these telescopes. Additionally, we predict the detectability of nebular lines for z > 10 galaxies, which will be found with JWST, the Wide-Field Infrared Survey Telescope (WFIRST) and First Light And Reionization Explorer (FLARE) (11 ≤ z ≤ 15). We conclude that the C IV 1549 Å, C III] 1909 Å, [O III] 4959/5007 Å and Hβ lines from even z ∼15 galaxies could be strong targets for TMT, E-ELT and JWST. We also find that magnification by gravitational lensing is of great help in detecting such high-z galaxies. According to our model, the C III] 1909 Å line in z > 9 galaxy candidates is detectable even using current facilities.... - Inoue, Akio K., Tamura, Yoichi, Matsuo, Hiroshi, Mawatari, Ken, Shimizu, Ikkoh, Shibuya, Takatoshi, Ota, Kazuaki, Yoshida, Naoki, Zackrisson, Erik, Kashikawa, Nobunari, Kohno, Kotaro, Umehata, Hideki, Hatsukade, Bunyo, Iye, Masanori, Matsuda, Yuichi, Okamoto, Takashi, Yamaguchi, YukiScience 352 6293 1559 - 1562 2016年06月 [査読有り][通常論文]
The physical properties and elemental abundances of the interstellar medium in galaxies during cosmic reionization are important for understanding the role of galaxies in this process. We report the Atacama Large Millimeter/submillimeter Array detection of an oxygen emission line at a wavelength of 88 micrometers from a galaxy at an epoch about 700 million years after the Big Bang. The oxygen abundance of this galaxy is estimated at about one-tenth that of the Sun. The nondetection of far-infrared continuum emission indicates a deficiency of interstellar dust in the galaxy. A carbon emission line at a wavelength of 158 micrometers is also not detected, implying an unusually small amount of neutral gas. These properties might allow ionizing photons to escape into the intergalactic medium.... - Makiya, Ryu, Enoki, Motohiro, Ishiyama, Tomoaki, Kobayashi, Masakazu A. R., Nagashima, Masahiro, Okamoto, Takashi, Okoshi, Katsuya, Oogi, Taira, Shirakata, HikariPublications of the Astronomical Society of Japan 68 2 2016年04月 [査読有り][通常論文]
We present a new cosmological galaxy formation model, ν2GC, as an updated version of our previous model νGC. We adopt the so-called "semi-analytic" approach, in which the formation history of dark matter halos is computed by N-body simulations, while the baryon physics such as gas cooling, star formation, and supernova feedback are simply modeled by phenomenological equations. Major updates of the model are as follows: (1) the merger trees of dark matter halos are constructed in state-of-the-art N-body simulations, (2) we introduce the formation and evolution process of supermassive black holes and the suppression of gas cooling due to active galactic nucleus (AGN) activity, (3) we include heating of the intergalactic gas by the cosmic UV background, and (4) we tune some free parameters related to the astrophysical processes using a Markov chain Monte Carlo method. Our N-body simulations of dark matter halos have unprecedented box size and mass resolution (the largest simulation contains 550 billion particles in a 1.12 Gpc h-1 box), enabling the study of much smaller and rarer objects. The model was tuned to fit the luminosity functions of local galaxies and mass function of neutral hydrogen. Local observations, such as the Tully-Fisher relation, the size-magnitude relation of spiral galaxies, and the scaling relation between the bulge mass and black hole mass were well reproduced by the model. Moreover, the model also reproduced well the cosmic star formation history and redshift evolution of rest-frame K-band luminosity functions. The numerical catalog of the simulated galaxies and AGNs is publicly available on the web.... - Tanaka, Satoshi, Yoshikawa, Kohji, Okamoto, Takashi, Hasegawa, KenjiPublications of the Astronomical Society of Japan 67 4 2015年08月 [査読有り][通常論文]
We present a new numerical scheme to solve the transfer of diffuse radiation on three-dimensional mesh grids which is efficient on processors with highly parallel architecture such as recently popular GPUs and CPUs with multi- and many-core architectures. The scheme is based on the ray-tracing method and the computational cost is proportional to N_m^{ 5/3}, where Nm is the number of mesh grids, and is devised to compute the radiation transfer along each light-ray completely in parallel, with appropriate grouping of the light-rays. We find that the performance of our scheme scales well with the number of adopted CPU cores and GPUs, and also that our scheme is nicely parallelized on a multi-node system by adopting the multiple wave front scheme, and the performance scales well with the amount of computational resources. As numerical tests to validate our scheme and to give a physical criterion for the angular resolution of our ray-tracing scheme, we perform several numerical simulations of the photoionization of neutral hydrogen gas by ionizing radiation sources without the "on-the-spot" approximation, in which the transfer of diffuse radiation by radiative recombination is incorporated in a self-consistent manner.... - Okamoto, Takashi, Isoe, Mari, Habe, AsaoPublications of the Astronomical Society of Japan 67 4 2015年08月 [査読有り][通常論文]
We investigate the evolution of two bars formed in fully self-consistent hydrodynamic simulations of the formation of Milky Way-mass galaxies. One galaxy shows higher central mass concentration and has a longer and stronger bar than the other at z = 0. The stronger bar evolves by transferring its angular momentum mainly to the dark halo. Consequently the rotation speed of the bar decreases with time, while the amplitude of the bar increases with time. These features qualitatively agree with the results obtained by idealized simulations. The pattern speed of the stronger bar largely goes up and down within a half revolution in its early evolutionary stage. These oscillations occur when the bar is misaligned with the m = 4 Fourier component. These oscillations correlate with the oscillations in the triaxiality of the dark matter halo, but differently from the way identified by idealized simulations. The amplitude of the weaker bar does not increase despite the fact that its rotation slows down with time. This result contradicts what is expected from idealized simulations and is caused by the decline of the central density associated with the mass loss and feedback from the stellar populations. The amplitude of the weaker bar is further weakened by the angular momentum injection from interactions with stellar clumps in the disk. In the both galaxies, the bars are terminated around the 4:1 resonance.... - Shirakata, H., Okamoto, T., Enoki, M., Nagashima, M., Kobayashi, M. A. R., Ishiyama, T., Makiya, R.Monthly Notices of the Royal Astronomical Society 450 1 L6 - L10 2015年06月 [査読有り][通常論文]
We have investigated effects of dust attenuation on quasar luminosity functions at z ∼ 2 using a semi-analytic galaxy formation model combined with a large cosmological N-body simulation. We estimate the dust attenuation of quasars self-consistently with that of galaxies by considering the dust in their host bulges. We find that the luminosity of the bright quasars is strongly dimmed by the dust attenuation, ∼2 mag in the B-band. Assuming the empirical bolometric corrections for active galactic nuclei (AGNs) by Marconi et al., we find that this dust attenuation is too strong to explain the B-band and X-ray quasar luminosity functions simultaneously. We consider two possible mechanisms that weaken the dust attenuation. As such a mechanism, we introduce a time delay for AGN activity, that is, gas fuelling to a central black hole starts sometime after the beginning of the starburst induced by a major merger. The other is the anisotropy in the dust distribution. We find that in order to make the dust attenuation of the quasars negligible, either the gas accretion into the black holes has to be delayed at least three times the dynamical time-scale of their host bulges or the dust covering factor is as small as ∼0.1.... - Hikari Shirakata, Takashi Okamoto, Motohiro Enoki, Masahiro Nagashima, Masakazu A. R. Kobayashi, Tomoaki Ishiyama, Ryu MakiyaMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 450 1 L6 - L10 2015年06月 [査読有り][通常論文]
We have investigated effects of dust attenuation on quasar luminosity functions at z similar to 2 using a semi-analytic galaxy formation model combined with a large cosmological N-body simulation. We estimate the dust attenuation of quasars self-consistently with that of galaxies by considering the dust in their host bulges. We find that the luminosity of the bright quasars is strongly dimmed by the dust attenuation, similar to 2 mag in the B-band. Assuming the empirical bolometric corrections for active galactic nuclei (AGNs) by Marconi et al., we find that this dust attenuation is too strong to explain the B-band and X-ray quasar luminosity functions simultaneously. We consider two possible mechanisms that weaken the dust attenuation. As such a mechanism, we introduce a time delay for AGN activity, that is, gas fuelling to a central black hole starts sometime after the beginning of the starburst induced by a major merger. The other is the anisotropy in the dust distribution. We find that in order to make the dust attenuation of the quasars negligible, either the gas accretion into the black holes has to be delayed at least three times the dynamical time-scale of their host bulges or the dust covering factor is as small as similar to 0.1. - Okamoto, Takashi, Shimizu, Ikkoh, Yoshida, NaokiPublications of the Astronomical Society of Japan 66 4 2014年07月 [査読有り][通常論文]
We present cosmological hydrodynamic simulations performed to study the evolution of galaxy population. The simulations follow timed release of mass, energy, and metals by stellar evolution and employ phenomenological treatments of supernova feedback, pre-supernova feedback modeled as feedback by radiation pressure from massive stars, and quenching of gas cooling in large halos. We construct the fiducial model so that it reproduces the observationally estimated galaxy stellar mass functions and the relationships between the galaxy stellar mass and the host halo mass from z = 4 to 0. We find that the fiducial model constructed this way naturally explains the cosmic star-formation history, the galaxy downsizing, and the star-formation rate and metallicity of the star-forming galaxies. The simulations without the quenching of the gas cooling in large halos overproduce massive galaxies at z < 2 and fail to reproduce galaxy downsizing. The simulations that do not employ the radiation pressure feedback from young stars predict too strong redshift evolution of the mass-metallicity relation. Furthermore, the slope of the relation becomes too steep at low redshift without the radiation pressure feedback. The metallicity dependence in the radiation pressure feedback is a key to explain the observed mass-metallicity relation. These facts indicate that these two processes in addition to supernova feedback are essential for galaxy evolution. Our simple phenomenological model is suitable to construct a mock galaxy sample to study physical properties of observed galaxy populations.... - Shimizu, Ikkoh, Inoue, Akio K., Okamoto, Takashi, Yoshida, NaokiMonthly Notices of the Royal Astronomical Society 440 1 731 - 745 2014年05月 [査読有り][通常論文]
We have performed a large cosmological hydrodynamics simulation tailored to the deep survey with the Hubble Space Telescope made in 2012, the so-called UDF12 campaign. After making a light-cone output, we have applied the same colour-selection criteria as the UDF12 campaign to select galaxies from our simulation, and then, have examined the physical properties of them as a proxy of the real observed UDF12 galaxies at z > 7. As a result, we find that the halo mass is almost linearly proportional to the observed ultraviolet (UV) luminosity (4 × 1011M☉ at MUV = -21). The dust attenuation and UV slope β well correlates with the observed UV luminosity, which is consistent with observations quantitatively. The star formation rate (SFR) is also linearly proportional to the stellar mass and the specific SFR shows only a weak dependence on the mass. We also find an increasing star formation history with a time-scale of ∼100 Myr in the high-z galaxies. An average metallicity weighted by the Lyman continuum luminosity reaches up to >0.1 Solar even at z ∼ 10, suggesting a rapid metal enrichment. We also expect ≥0.1 mJy at 350 GHz of the dust thermal emission from the galaxies with H160 ≤ 27, which can be detectable with the Atacama Large Millimetre-sub-millimetre Array. The galaxies selected by the UDF12 survey contribute to only 52-12 per cent of the cosmic SFR density from z ∼ 7 to z ∼ 10, respectively. The James Webb Space Telescope will push the detection fraction up to 77-72 per cent.... - Inoue, A. K., Shimizu, I., Tamura, Y., Matsuo, H., Okamoto, T., Yoshida, N.The Astrophysical Journal 2014年01月 [査読有り][通常論文]
We investigate the potential use of nebular emission lines in the rest-frame far-infrared (FIR) for determining spectroscopic redshift of z > 8 galaxies with the Atacama Large Millimeter/submillimeter Array (ALMA). After making a line emissivity model as a function of metallicity, especially for the [O III] 88 μm line which is likely to be the strongest FIR line from H II regions, we predict the line fluxes from high-z galaxies based on a cosmological hydrodynamics simulation of galaxy formation. Since the metallicity of galaxies reaches at ~0.2 Z ☉ even at z > 8 in our simulation, we expect the [O III] 88 μm line as strong as 1.3 mJy for 27 AB objects, which is detectable at a high significance by <1 hr integration with ALMA. Therefore, the [O III] 88 μm line would be the best tool to confirm the spectroscopic redshifts beyond z = 8.... - Tanikawa, Ataru, Yoshikawa, Kohji, Nitadori, Keigo, Okamoto, TakashiNew Astronomy 19 74 - 88 2013年02月 [査読有り][通常論文]
We have developed a numerical software library for collisionless N-body simulations named "Phantom-GRAPE" which highly accelerates force calculations among particles by use of a new SIMD instruction set extension to the x86 architecture, Advanced Vector eXtensions (AVX), an enhanced version of the Streaming SIMD Extensions (SSE). In our library, not only the Newton's forces, but also central forces with an arbitrary shape f(r), which has a finite cutoff radius rcut (i.e. f(r)=0 at r>rcut), can be quickly computed. In computing such central forces with an arbitrary force shape f(r), we refer to a pre-calculated look-up table. We also present a new scheme to create the look-up table whose binning is optimal to keep good accuracy in computing forces and whose size is small enough to avoid cache misses. Using an Intel Core i7-2600 processor, we measure the performance of our library for both of the Newton's forces and the arbitrarily shaped central forces. In the case of Newton's forces, we achieve 2×109 interactions per second with one processor core (or 75 GFLOPS if we count 38 operations per interaction), which is 20 times higher than the performance of an implementation without any explicit use of SIMD instructions, and 2 times than that with the SSE instructions. With four processor cores, we obtain the performance of 8×109 interactions per second (or 300 GFLOPS). In the case of the arbitrarily shaped central forces, we can calculate 1×109 and 4×109 interactions per second with one and four processor cores, respectively. The performance with one processor core is 6 times and 2 times higher than those of the implementations without any use of SIMD instructions and with the SSE instructions. These performances depend only weakly on the number of particles, irrespective of the force shape. It is good contrast with the fact that the performance of force calculations accelerated by graphics processing units (GPUs) depends strongly on the number of particles. Substantially weak dependence of the performance on the number of particles is suitable to collisionless N-body simulations, since these simulations are usually performed with sophisticated N-body solvers such as Tree- and TreePM-methods combined with an individual timestep scheme. We conclude that collisionless N-body simulations accelerated with our library have significant advantage over those accelerated by GPUs, especially on massively parallel environments.... - Kawata, D., Okamoto, T., Gibson, B. K., Barnes, D. J., Cen, R.Monthly Notices of the Royal Astronomical Society 2013年01月 [査読有り]
We adapt a modern scheme of smoothed particle hydrodynamics (SPH) to our tree N-body/SPH galactic chemodynamics code gcd+. The applied scheme includes implementations of the artificial viscosity switch and artificial thermal conductivity proposed by Morris & Monaghan, Rosswog & Price and Price to model discontinuities and Kelvin-Helmholtz instabilities more accurately. We first present hydrodynamics test simulations and contrast the results to runs undertaken without artificial viscosity switch or thermal conduction. In addition, we also explore the different levels of smoothing by adopting larger or smaller smoothing lengths, i.e. a larger or smaller number of neighbour particles, Nnb. We demonstrate that the new version of gcd+ is capable of modelling Kelvin-Helmholtz instabilities to a similar level as the mesh code, athena. From the Gresho vortex, point-like explosion and self-similar collapse tests, we conclude that setting the smoothing length to keep Nnb as high as ∼58 is preferable to adopting smaller smoothing lengths. We present our optimized parameter sets from the hydrodynamics tests.... - Takashi OkamotoMonthly Notices of the Royal Astronomical Society 428 1 718 - 728 2013年01月 [査読有り][通常論文]
More than half of nearby disc galaxies have pseudo-bulges, instead of classical bulges that are thought to be end-products of galaxy mergers. Pseudo-bulges are presumed to develop over time as a result of the secular evolution of galaxy discs. We report on simulations of galaxy formation, in which two disc galaxies with discy pseudo-bulges have formed. Based on the profile decomposition, the bulge-to-total mass ratio of the simulated galaxies is 0.6 for one galaxy and 0.3 for the other. We find that the main formation mechanism of the pseudo-bulges in our simulations is not the secular evolution of discs but high-redshift starbursts. The progenitors of the pseudo-bulges form as high-redshift discs with small scalelengths by the rapid supply of low angular momentum gas. By redshift 2, before the main disc formation, pseudo-bulge formation has largely completed in terms of mass. The secular evolution, such as bar instability, accounts for about 30 per cent of the bulge mass for one galaxy and only ∼13 per cent for the other. However, it does affect the final shape and kinematic properties of the pseudo-bulges.... - Shimizu, Ikkoh, Yoshida, Naoki, Okamoto, TakashiMonthly Notices of the Royal Astronomical Society 427 4 2866 - 2875 2012年12月 [査読有り][通常論文]
We use large cosmological smoothed particle hydrodynamics simulations to study the formation and evolution of submillimetre galaxies (SMGs). In our previous work, we studied the statistical properties of ultraviolet-selected star-forming galaxies at high redshifts. We populate the same cosmological simulations with SMGs by calculating the reprocessing of stellar light by dust grains into far-infrared to millimetre wavebands in a self-consistent manner. We generate light-cone outputs to compare directly the statistical properties of the simulated SMGs with available observations. Our model reproduces the submillimetre source number counts and the clustering amplitude. We show that bright SMGs with flux S > 1 mJy reside in haloes with masses of ∼1013 M☉ and have stellar masses greater than 1011 M☉. The angular cross-correlation between the SMGs and Lyman α emitters is significantly weaker than that between the SMGs and Lyman-break galaxies. The cross-correlation is also weaker than the autocorrelation of the SMGs. The redshift distribution of the SMGs shows a broad peak at z ∼ 2, where bright SMGs contribute significantly to the global cosmic star formation rate density. Our model predicts that there are hundreds of SMGs with S > 0.1 mJy at z > 5 per 1 deg2 field. Such SMGs can be detected by ALMA.... - Scannapieco, C., Wadepuhl, M., Parry, O. H., Navarro, J. F., Jenkins, A., Springel, V., Teyssier, R., Carlson, E., Couchman, H. M. P., Crain, R. A., Dalla Vecchia, C., Frenk, C. S., Kobayashi, C., Monaco, P., Murante, G., Okamoto, T., Quinn, T., Schaye, J., Stinson, G. S., Theuns, T., Wadsley, J., White, S. D. M., Woods, R.Monthly Notices of the Royal Astronomical Society 423 2 1726 - 1749 2012年06月 [査読有り][通常論文]
We compare the results of various cosmological gas-dynamical codes used to simulate the formation of a galaxy in the Λ cold dark matter structure formation paradigm. The various runs (13 in total) differ in their numerical hydrodynamical treatment [smoothed particle hydrodynamics (SPH), moving mesh and adaptive mesh refinement] but share the same initial conditions and adopt in each case their latest published model of gas cooling, star formation and feedback. Despite the common halo assembly history, we find large code-to-code variations in the stellar mass, size, morphology and gas content of the galaxy at z= 0, due mainly to the different implementations of star formation and feedback. Compared with observation, most codes tend to produce an overly massive galaxy, smaller and less gas rich than typical spirals, with a massive bulge and a declining rotation curve. A stellar disc is discernible in most simulations, although its prominence varies widely from code to code. There is a well-defined trend between the effects of feedback and the severity of the disagreement with observed spirals. In general, models that are more effective at limiting the baryonic mass of the galaxy come closer to matching observed galaxy scaling laws, but often to the detriment of the disc component. Although numerical convergence is not particularly good for any of the codes, our conclusions hold at two different numerical resolutions. Some differences can also be traced to the different numerical techniques; for example, more gas seems able to cool and become available for star formation in grid-based codes than in SPH. However, this effect is small compared to the variations induced by different feedback prescriptions. We conclude that state-of-the-art simulations cannot yet uniquely predict the properties of the baryonic component of a galaxy, even when the assembly history of its host halo is fully specified. Developing feedback algorithms that can effectively regulate the mass of a galaxy without hindering the formation of high angular momentum stellar discs remains a challenge.... - Matsui, Hidenori, Saitoh, Takayuki R., Makino, Junichiro, Wada, Keiichi, Tomisaka, Kohji, Kokubo, Eiichiro, Daisaka, Hiroshi, Okamoto, Takashi, Yoshida, NaokiThe Astrophysical Journal 746 1 26 2012年02月 [査読有り][通常論文]
Ultraluminous infrared galaxies (ULIRGs) with multiple (>=3) nuclei are frequently observed. It has been suggested that these nuclei are produced by multiple major mergers of galaxies. The expected rate of such mergers is, however, too low to reproduce the observed number of ULIRGs with multiple nuclei. We have performed high-resolution simulations of the merging of two gas-rich disk galaxies. We found that extremely massive and compact star clusters form from the strongly disturbed gas disks after the first or second encounter between the galaxies. The mass of such clusters reaches ~108 M ☉ and their half-mass radii are 20-30 pc. Since these clusters consist of young stars, they appear as several bright cores in the galactic central region (~kpc). The peak luminosity of these clusters reaches ~10% of the total luminosity of the merging galaxy. These massive and compact clusters are consistent with the characteristics of the observed multiple nuclei in ULIRGs. Multiple mergers are not necessary to explain multiple nuclei in ULIRGs.... - Parry, O. H., Eke, V. R., Frenk, C. S., Okamoto, T.Monthly Notices of the Royal Astronomical Society 419 4 3304 - 3318 2012年02月 [査読有り][通常論文]
We investigate the formation and evolution of satellite galaxies using smoothed particle hydrodynamics (SPH) simulations of a Milky Way (MW) like system, focusing on the best resolved examples, analogous to the classical MW satellites. Comparing with a pure dark matter simulation, we find that the condensation of baryons has had a relatively minor effect on the structure of the satellites' dark matter haloes. The stellar mass that forms in each satellite agrees relatively well over three levels of resolution (a factor of ∼64 in particle mass) and scales with (sub)halo mass in a similar way in an independent semi-analytical model. Our model provides a relatively good match to the average luminosity function of the MW and M31. To establish whether the potential wells of our satellites are realistic, we measure their masses within observationally determined half-light radii, finding that they have somewhat higher mass-to-light ratios than those derived for the MW dSphs from stellar kinematic data; the most massive examples are most discrepant. A statistical test yields an ∼6 per cent probability that the simulated and observationally derived distributions of masses are consistent. If the satellite population of the MW is typical, our results could imply that feedback processes not properly captured by our simulations have reduced the central densities of subhaloes, or that they initially formed with lower concentrations, as would be the case, for example, if the dark matter were made of warm, rather than cold particles.... - Okamoto, Takashi, Yoshikawa, Kohji, Umemura, MasayukiMonthly Notices of the Royal Astronomical Society 419 4 2855 - 2866 2012年02月 [査読有り][通常論文]
We present two types of numerical prescriptions that accelerate the radiative transfer calculation around point sources within a three-dimensional Cartesian grid by using the oct-tree structure for the distribution of radiation sources. In one prescription, distant radiation sources are grouped as a bright extended source when the group's angular size, θs, is smaller than a critical value, θcrit, and radiative transfer is solved on supermeshes whose angular size is similar to that of the group of sources. The supermesh structure is constructed by coarse-graining the mesh structure. With this method, the computational time scales with Nmlog (Nm)log (Ns), where Nm and Ns are the number of meshes and that of radiation sources, respectively. While this method is very efficient, it inevitably overestimates the optical depth when a group of sources acts as an extended powerful radiation source and affects distant meshes. In the other prescription, a distant group of sources is treated as a bright point source ignoring the spatial extent of the group, and the radiative transfer is solved on the meshes rather than the supermeshes. This prescription is simply a grid-based version of START by Hasegawa & Umemura and yields better results in general with slightly more computational cost [?] than the supermesh prescription. Our methods can easily be implemented to any grid-based hydrodynamic codes and are well suited to adaptive mesh refinement methods.... - Tanikawa, Ataru, Yoshikawa, Kohji, Okamoto, Takashi, Nitadori, KeigoNew Astronomy 17 2 82 - 92 2012年02月 [査読有り][通常論文]
We present a high-performance N-body code for self-gravitating collisional systems accelerated with the aid of a new SIMD instruction set extension of the x86 architecture: Advanced Vector eXtensions (AVX), an enhanced version of the Streaming SIMD Extensions (SSE). With one processor core of Intel Core i7-2600 processor (8 MB cache and 3.40 GHz) based on Sandy Bridge micro-architecture, we implemented a fourth-order Hermite scheme with individual timestep scheme ( Makino and Aarseth, 1992), and achieved the performance of ∼20 giga floating point number operations per second (GFLOPS) for double-precision accuracy, which is two times and five times higher than that of the previously developed code implemented with the SSE instructions ( Nitadori et al., 2006b), and that of a code implemented without any explicit use of SIMD instructions with the same processor core, respectively. We have parallelized the code by using so-called NINJA scheme ( Nitadori et al., 2006a), and achieved ∼90 GFLOPS for a system containing more than N = 8192 particles with 8 MPI processes on four cores. We expect to achieve about 10 tera FLOPS (TFLOPS) for a self-gravitating collisional system with N ∼ 10 5 on massively parallel systems with at most 800 cores with Sandy Bridge micro-architecture. This performance will be comparable to that of Graphic Processing Unit (GPU) cluster systems, such as the one with about 200 Tesla C1070 GPUs ( Spurzem et al., 2010). This paper offers an alternative to collisional N-body simulations with GRAPEs and GPUs.... - SHIMIZU IkkohWiley-Blackwell for the Royal Astronomical SocietyMonthly Notices of the Royal Astronomical Society 418 4 2273 - 2282 2011年12月 [査読有り][通常論文]
We use very large cosmological smoothed particle hydrodynamics simulations to study the properties of high-redshift Lyman α emitters (LAEs). We identify star-forming galaxies at z= 3.1 in a cosmological volume of 100 h-1 Mpc on a side. We develop a phenomenological model of absorption, scattering and escape of Lyman α photons on the assumption that the clumpiness of the interstellar medium in a galaxy is correlated with the larger scale substructure richness. The radiative transfer effect proposed by Neufeld allows a large fraction of Lyman α photons to escape from a clumpy galaxy even if it contains a substantial amount of dust. Our model reproduces, for the first time, all of the following observed properties of LAEs at z= 3.1: the angular correlation function, ultraviolet (UV) and Lyman α luminosity functions and the equivalent width distribution. A simple model that takes only dust absorption into account fails in reproducing LAEs that are also bright in UV, suggesting that the kind of effect we consider is needed. Our model predicts a bimodal age distribution for LAEs. Most of the galaxies with large Lyman α equivalent widths are young, whereas there are also old, massive and dusty LAEs, similar to recently found high-redshift LAEs. The large LAEs have escape fractions of Lyman α photons of fesc∼ 0.05-0.1.... - Okamoto, Takashi, Frenk, Carlos S., Jenkins, Adrian, Theuns, TomMonthly Notices of the Royal Astronomical Society 406 1 208 - 222 2010年07月 [査読有り][通常論文]
We investigate the properties of satellite galaxies in cosmological N-body/smooth particle hydrodynamics simulations of galaxy formation in Milky Way-sized haloes. Because of their shallow potential wells, satellite galaxies are very sensitive to heating processes which affect their gas content. Their properties can therefore be used to constrain the nature of feedback processes that regulate galaxy formation. In our simulations, we assume that all the energy produced by supernovae is used as kinetic energy to drive galactic winds. Several of our simulations produce bright, disc-dominated galaxies. We find that wind models in which the wind speed, vw, is proportional to the local velocity dispersion of the dark matter, (and thus the wind mass-loading, ), make star formation in satellites sporadic, reproduce the observed satellite luminosity function reasonably well (down to Mv=-7) and match the luminosity-metallicity relation observed in the Local Group satellites. By contrast, models that assume a constant wind speed overproduce faint satellites and predict an incorrect luminosity-metallicity relation. Our simulations therefore suggest that the feedback processes that operate on the scale of satellite galaxies should generate galactic outflows whose mass-loading varies inversely with the depth of the potential.... - Bett, Philip, Eke, Vincent, Frenk, Carlos S., Jenkins, Adrian, Okamoto, TakashiMonthly Notices of the Royal Astronomical Society 404 3 1137 - 1156 2010年05月 [査読有り][通常論文]
We investigate the magnitude and internal alignment of the angular momentum of cold dark matter haloes in simulations with and without baryons. We analyse the cumulative angular momentum profiles of hundreds of thousands of well-resolved haloes in the Millennium Simulation of Springel et al. and in a smaller, but higher resolution, simulation, in total spanning 5 orders of magnitude in mass. For haloes of a given mass, the median specific angular momentum increases with radius as j(<=r) ~ r. The direction of the vector varies considerably with radius: the median angle between the inner (<~0.25Rvir) and total (<=Rvir) angular momentum vectors is about 25°. To investigate how baryons affect halo spin, we use another high-resolution simulation, which includes gas cooling, star formation and feedback. This simulation produces a sample of galaxies with a realistic distribution of disc-to-total ratios, D/T: two-thirds of the galaxies have D/T > 0.5 in the B band. The formation of the galaxy spins up the dark matter within 0.1Rvir such that the specific halo angular momentum increases by ~50 per cent in the median. The dark matter angular momentum becomes better aligned, but there remains a broad distribution of (mis-)alignments between the halo and the central galaxy, with a median angle between their angular momenta of ~30°. Galaxies have a range of orientations relative to the shape of the halo: half of them have their minor axes misaligned by more than 45°, although only about 10 per cent of the galaxies lie within 30° of the plane perpendicular to the major axis of their halo. Finally, we align a sample of haloes according to the orientation of their galaxies and stack the projected mass distributions. Although the individual haloes are significantly aspherical, galaxy-halo misalignments produce a stacked mass distribution that cannot be distinguished from circular. If the lack of alignment found in our simulations is realistic, it will be extremely difficult for weak lensing studies to measure the ellipticity of cold dark matter haloes using this technique.... - Saitoh, Takayuki R., Daisaka, Hiroshi, Kokubo, Eiichiro, Makino, Junichiro, Okamoto, Takashi, Tomisaka, Kohji, Wada, Keiichi, Yoshida, NaokiGalaxy Wars: Stellar Populations and Star Formation in Interacting Galaxies 423 185 - + 2010年 [査読有り][通常論文]
- Okamoto, Takashi, Frenk, Carlos S.Monthly Notices of the Royal Astronomical Society 399 1 L174 - L178 2009年10月 [査読有り][通常論文]
We study the formation histories and present-day structure of satellite galaxies formed in a high-resolution hydrodynamic simulation of a Milky Way like galaxy. The simulated satellites span nearly four orders of magnitude in luminosity but have a very similar mass within their inner 600 pc, ~3 × 107Msolar, with very little scatter. This result is in agreement with the recent measurements for dwarf spheroidal galaxies (dSphs) in the Milky Way by Strigari et al. In our simulations, a preferred mass scale arises naturally from the effects of the early re-ionization of gas. These impose a sharp threshold of ~12kms-1 on the circular velocity of haloes which can cool gas and make stars. At the present day, subhaloes that host satellites as luminous as the classical Milky Way dwarfs (LV >= 2.6 × 105Lsolar) have typically grown to have circular velocities of >~20 km s-1. There are, however, subhaloes with similar circular velocities today which were, nevertheless, below threshold at re-ionization and thus remain dark. Star formation in above-threshold haloes is truncated when the halo is accreted into the main galaxy progenitor. Thus, many properties of today's dwarf satellites such as their luminosity and star formation rate are set by their accretion time.... - Saitoh, Takayuki R., Daisaka, Hiroshi, Kokubo, Eiichiro, Makino, Junichiro, Okamoto, Takashi, Tomisaka, Kohji, Wada, Keiichi, Yoshida, NaokiPublications of the Astronomical Society of Japan 61 3 481 - 486 2009年06月 [査読有り][通常論文]
We investigated the evolution of interacting disk galaxies using high-resolution N-body/SPH simulations, while taking into account the multiphase nature of the interstellar medium. In our high-resolution simulations, a large-scale starburst occurred naturally at the collision interface between two gas disks at the first encounter, resulting in the formation of star clusters. This is consistent with observations of interacting galaxies. The probability distribution function (PDF) of the gas density showed a clear change during the galaxy-galaxy encounter. The compression of gas at the collision interface between the gas disks first appears as an excess at nH ∼ 10cm-3 in the PDF, and then it moves to higher densities (nH gtrsim 100 cm-3) in a few times 107years, where starburst takes place. After the starburst, the PDF goes back to a quasi-steady state. These results give a simple picture of starburst phenomena in galaxy-galaxy encounters.... - Okamoto, Takashi, Gao, Liang, Theuns, TomMonthly Notices of the Royal Astronomical Society 390 3 920 - 928 2008年11月 [査読有り][通常論文]
We perform cosmological hydrodynamic simulations to determine to what extent galaxies lose their gas due to photoheating from an ionizing background. We find that the characteristic mass at which haloes on average have lost half of their baryons is Mc ~ 6.5 × 109h-1Msolar at z = 0, which corresponds to a circular velocity of 25kms-1. This is significantly lower than the filtering mass obtained by the linear theory, which is often used in semi-analytical models of galaxy formation. We demonstrate it is the gas temperature at the virial radius which determines whether a halo can accrete gas. A simple model that follows the merger history of the dark matter progenitors, and where gas accretion is not allowed when this temperature is higher than the virial temperature of the halo, reproduces the results from the simulation remarkably well. This model can be applied to any reionization history, and is easy to incorporate in semi-analytical models.... - Saitoh, Takayuki R., Daisaka, Hiroshi, Kokubo, Eiichiro, Makino, Junichiro, Okamoto, Takashi, Tomisaka, Kohji, Wada, Keiichi, Yoshida, NaokiPublications of the Astronomical Society of Japan 60 4 667 - 681 2008年08月 [査読有り][通常論文]
We performed 3-dimensional N-body/SPH simulations to study how mass resolution and other model parameters such as the star formation efficiency parameter, C* and the threshold density, nth affect structures of the galactic gaseous/stellar disk in a static galactic potential. We employ 10^6 - 10^7 particles to resolve a cold and dense (T < 100 K & n_H > 100 cm^{-3}) phase. We found that structures of the ISM and the distribution of young stars are sensitive to the assumed nth. High-nth models with nth = 100 cm^{-3} yield clumpy multi-phase features in the ISM. Young stars are distributed in a thin disk of which half-mass scale height is 10 - 30 pc. In low-nth models with nth = 0.1 cm^{-3}, the stellar disk is found to be several times thicker, and the gas disk appears smoother than the high-nth models. A high-resolution simulation with high-nth is necessary to reproduce the complex structure of the gas disk. The global properties of the model galaxies in low-nth models, such as star formation histories, are similar to those in the high-nth models when we tune the value of C* so that they reproduce the observed relation between surface gas density and surface star formation rate density. We however emphasize that high-nth models automatically reproduce the relation, regardless of the values of C*. The ISM structure, phase distribution, and distributions of young star forming region are quite similar between two runs with values of C* which differ by a factor of 15. We also found that the timescale of the flow from n_H ~1 cm^{-3} to n_H > 100 cm^{-3} is about 5 times as long as the local dynamical time and is independent of the value of C*. The use of a high-nth criterion for star formation in high-resolution simulations makes numerical models fairy insensitive to the modelling of star formation. (Abridged)... - Zavala, Jesus, Okamoto, Takashi, Frenk, Carlos S.Monthly Notices of the Royal Astronomical Society 387 1 364 - 370 2008年06月 [査読有り][通常論文]
We investigate the evolution of angular momentum in simulations of galaxy formation in a cold dark matter universe. We analyse two model galaxies generated in the N-body/hydrodynamic simulations of Okamoto et al. Starting from identical initial conditions, but using different assumptions for the baryonic physics, one of the simulations produced a bulge-dominated galaxy and the other one a disc-dominated galaxy. The main difference is the treatment of star formation and feedback, both of which were designed to be more efficient in the disc-dominated object. We find that the specific angular momentum of the disc-dominated galaxy tracks the evolution of the angular momentum of the dark matter halo very closely: the angular momentum grows as predicted by linear theory until the epoch of maximum expansion and remains constant thereafter. By contrast, the evolution of the angular momentum of the bulge-dominated galaxy resembles that of the central, most bound halo material: it also grows at first according to linear theory, but 90 per cent of it is rapidly lost as pre-galactic fragments, into which gas had cooled efficiently, merge, transferring their orbital angular momentum to the outer halo by tidal effects. The disc-dominated galaxy avoids this fate because the strong feedback reheats the gas, which accumulates in an extended hot reservoir and only begins to cool once the merging activity has subsided. Our analysis lends strong support to the classical theory of disc formation whereby tidally torqued gas is accreted into the centre of the halo conserving its angular momentum.... - Okamoto, Takashi, Nemmen, Rodrigo S., Bower, Richard G.Monthly Notices of the Royal Astronomical Society 385 1 161 - 180 2008年03月 [査読有り][通常論文]
In this paper, we present a new implementation of feedback due to active galactic nuclei (AGN) in cosmological simulations of galaxy formation. We assume that a fraction of jet energy, which is generated by an AGN, is transferred to the surrounding gas as thermal energy. Combining a theoretical model of mass accretion on to black holes with a multiphase description of star-forming gas, we self-consistently follow evolution of both galaxies and their central black holes. The novelty in our model is that we consider two distinct accretion modes: standard radiatively efficient thin accretion discs and radiatively inefficient accretion flows which we will generically refer to as RIAFs; motivated by theoretical models for jet production in accretion discs, we assume that only the RIAF is responsible for the AGN feedback. The focus of this paper is to investigate the interplay between galaxies and their central black holes during the formation of a disc galaxy. We find that, after an initial episode of bursting star formation, the accretion rate on to the central black hole drops so that the accretion disc switches to a RIAF structure. At this point, the feedback from the AGN becomes efficient and slightly suppresses star formation in the galactic disc and almost completely halts star formation in the bulge. This suppression of the star formation regulates mass accretion on to the black hole and associated AGN feedback. As a result, the nucleus becomes a stochastically fuelled low-luminosity AGN (Seyfert galaxy) with recurrent short-lived episodes of activity after the star bursts. During the `on' events, the AGN produces reasonably powerful jets (radio-loud state) and is less luminous than the host galaxy, while in the `off' phase, the nucleus is inactive and `radio quiet'. Our model predicts several properties of the low-luminosity AGN including the bolometric luminosity, jet powers, the effect on kpc scale of the radio jet and the AGN lifetime, which are in broad agreement with observations of Seyfert galaxies and their radio activity. We also find that the ratios between the central black hole mass and the mass of the host spheroid at z = 0 are ~10-3 regardless of the strength of either supernova feedback or AGN feedback because the radiation drag model directly relates the star formation activity in the Galactic Centre and the mass accretion rate on to the central black hole.... - Booth, C. M., Theuns, Tom, Okamoto, TakashiMonthly Notices of the Royal Astronomical Society 376 4 1588 - 1610 2007年04月 [査読有り][通常論文]
We describe a numerical implementation of star formation in disc galaxies, in which the conversion of cooling gas to stars in the multiphase interstellar medium is governed by the rate at which molecular clouds are formed and destroyed. In the model, clouds form from thermally unstable ambient gas and get destroyed by feedback from massive stars and thermal conduction. Feedback in the ambient phase cycles gas into a hot galactic fountain or wind. We model the ambient gas hydrodynamically using smoothed particle hydrodynamics. However, we cannot resolve the Jeans mass in the cold and dense molecular gas and, therefore, represent the cloud phase with ballistic particles that coagulate when colliding. We show that this naturally produces a multiphase medium with cold clouds, a warm disc, hot supernova bubbles and a hot, tenuous halo. Our implementation of this model is based on the GADGET N-body code. We illustrate the model by evolving an isolated Milky Way-like galaxy and study the properties of a disc formed in a rotating spherical collapse. Many observed properties of disc galaxies are reproduced well, including the molecular cloud mass spectrum, the molecular fraction as a function of radius, the Schmidt law, the stellar density profile and the appearance of a galactic fountain.... - Libeskind, Noam I., Cole, Shaun, Frenk, Carlos S., Okamoto, Takashi, Jenkins, AdrianMonthly Notices of the Royal Astronomical Society 374 1 16 - 28 2007年01月 [査読有り][通常論文]
We investigate the properties of satellite galaxies formed in N-body/SPH simulations of galaxy formation in the ΛCDM cosmology. The simulations include the main physical effects thought to be important in galaxy formation and, in several cases, produce realistic spiral discs. In total, a sample of nine galaxies of luminosity comparable to the Milky Way was obtained. At magnitudes brighter than the resolution limit, MV = -12, the luminosity function of the satellite galaxies in the simulations is in excellent agreement with data for the Local Group. The radial number density profile of the model satellites, as well as their gas fractions also match observations very well. In agreement with previous N-body studies, we find that the satellites tend to be distributed in highly flattened configurations whose major axis is aligned with the major axis of the (generally triaxial) dark halo. In two out of three systems with sufficiently large satellite populations, the satellite system is nearly perpendicular to the plane of the galactic disc, a configuration analogous to that observed in the Milk Way. The discs themselves are perpendicular to the minor axis of their host haloes in the inner parts, and the correlation between the orientation of the galaxy and the shape of the halo persists even out to the virial radius. However, in one case the disc's minor axis ends up, at the virial radius, perpendicular to the minor axis of the halo. The angular momenta of the galaxies and their host halo tend to be well aligned.... - Nagashima, Masahiro, Okamoto, TakashiThe Astrophysical Journal 643 2 863 - 880 2006年06月 [査読有り][通常論文]
We investigate chemical evolution in Milky Way-like galaxies based on the cold dark matter model in which cosmic structures form via hierarchical merging. We introduce chemical enrichment due to Type Ia supernovae (SNe Ia) into the Mitaka semianalytic galaxy formation model developed by Nagashima & Yoshii. For the first time we derive distributions of stellar metallicities and their ratios in Milky Way-like galaxies treating chemical enrichment due to SNe Ia in a hierarchical galaxy formation model self-consistently. As a first attempt, we assume all SNe Ia to have the same lifetime and assume instantaneous recycling for Type II supernovae (SNe II). We find that our model reproduces well the metal abundance ratio [O/Fe] against [Fe/H] and the iron metallicity distribution function in the solar neighborhood. This means that the so-called G dwarf problem is resolved by the hierarchical formation of galaxies, and a gas infall term introduced in traditional monolithic collapse models to solve this problem is well explained by the mixture of some physical processes such as hierarchical merging of dark halos, gas cooling, energy feedback, and injection of gas and metals into hot gas due to supernovae. We also discuss how physical processes affect the metal abundance ratio by varying the lifetime of SNe Ia, the star formation timescale, and the strength of supernova feedback. We find that the supernova feedback plays a key role among them, and therefore there is no one-to-one correspondence between star formation histories and stellar metallicity ratio distributions.... - Saitoh, Takayuki R., Koda, Jin, Okamoto, Takashi, Wada, Keiichi, Habe, AsaoThe Astrophysical Journal 640 1 22 - 30 2006年03月 [査読有り][通常論文]
Tidal disruption of dark matter halos around proto-globular clusters in the halo of a small galaxy is studied in the context of the hierarchical clustering scenario by using semicosmological N-body/SPH simulations assuming the standard cold dark matter model (Ω0=1). Our analysis on the formation and evolution of the galaxy and its substructures continues until z=2.0. In such a high-redshift universe, the Einstein-de Sitter universe is still a good approximation for the recently favored Λ-dominated universe, and thus our results do not depend on the choice of cosmology. In order to resolve small gravitationally bound clumps around galaxies and consider radiative cooling below T=104 K, we adopt a fine mass resolution (mSPH=1.12×103 Msolar). Because of the cooling, each clump immediately forms a ``core-halo'' structure that consists of a baryonic core and a dark matter halo. The tidal force from the host galaxy mainly strips the dark matter halo from clumps, and as a result, these clumps get dominated by baryons. Once a clump is captured by the host halo, its mass drastically decreases with each pericenter passage. At z=2, more than half of the clumps become baryon-dominated systems (baryon mass/total mass>0.5). Our results support the tidal evolution scenario of the formation of globular clusters and baryon-dominated dwarf galaxies in the context of the cold dark matter universe.... - Okamoto, Takashi, Eke, Vincent R., Frenk, Carlos S., Jenkins, AdrianMonthly Notices of the Royal Astronomical Society 363 4 1299 - 1314 2005年11月 [査読有り][通常論文]
We have performed hydrodynamic simulations of galaxy formation in a cold dark matter (ΛCDM) universe. We have followed galaxy formation in a dark matter halo, chosen to have a relatively quiet recent merger history, using different models for star formation and feedback. In all cases, we have adopted a multiphase description of the interstellar medium and modelled star formation in quiescent and burst modes. We have explored two triggers for starbursts - strong shocks and high gas density - allowing for the possibility that stars in the burst may form with a top-heavy initial mass function. We find that the final morphology of the galaxy is extremely sensitive to the modelling of star formation and feedback. Starting from identical initial conditions, galaxies spanning the entire range of Hubble types, with B-band disc-to-total luminosity ratios ranging from 0.2 to 0.9, can form in the same dark matter halo. Models in which starbursts are induced by high gas density (qualitatively similar to models in which feedback is produced by active galactic nuclei) generate energetic winds and result in galaxies with an early-type morphology. Models in which the starbursts are induced by strong shocks lead to extended discs. In this case, the feedback associated with the bursts suppresses the collapse of baryons in small haloes, helping to create a reservoir of hot gas that is available for cooling after z~= 1, following the bulk of the dynamical activity that builds up the halo. This gas then cools to form an extended, young stellar disc.... - Nagashima, Masahiro, Lacey, Cedric G., Okamoto, Takashi, Baugh, Carlton M., Frenk, Carlos S., Cole, ShaunMonthly Notices of the Royal Astronomical Society 363 1 L31 - L35 2005年10月 [査読有り][通常論文]
We investigate the metal enrichment of elliptical galaxies in the framework of hierarchical models of galaxy formation. The semi-analytical model we use, which has been used to study the metal enrichment of the intracluster medium (ICM) by Nagashima et al., includes the effects of flows of gas and metals both into and out of galaxies and the processes of metal enrichment due to both type Ia and type II supernovae. We adopt a solar neighbourhood initial mass function (IMF) for star formation in discs, but consider models in which starbursts have either a solar neighbourhood IMF or a top-heavy IMF. We find that the α-element abundance in ellipticals is consistent with observed values only if the top-heavy IMF is used. This result is consistent with our previous study on the metal enrichment of the ICM. We also discuss the abundance ratio of α elements to iron as a function of velocity dispersion and metallicity. We find that models with a top-heavy IMF match the α/Fe ratios observed in typical L* ellipticals, but none of the models reproduces the observed increase of α/Fe with velocity dispersion.... - Bailin, Jeremy, Kawata, Daisuke, Gibson, Brad K., Steinmetz, Matthias, Navarro, Julio F., Brook, Chris B., Gill, Stuart P. D., Ibata, Rodrigo A., Knebe, Alexander, Lewis, Geraint F., Okamoto, TakashiThe Astrophysical Journal 627 1 L17 - L20 2005年07月 [査読有り][通常論文]
Seven cosmological hydrodynamic simulations of disk galaxy formation are analyzed to determine the alignment of the disk within the dark matter halo and the internal structure of the halo. We find that the orientation of the outer halo, beyond ~0.1rvir, is unaffected by the presence of the disk. In contrast, the inner halo is aligned such that the halo minor axis aligns with the disk axis. The relative orientations of these two regions of the halo are uncorrelated. The alignment of the disk and inner halo appears to take place simultaneously through their joint evolution. The lack of connection between these two regions of the halo should be taken into account when modeling tidal streams in the halos of disk galaxies and when calculating intrinsic alignments of disk galaxies based on the properties of dark matter halos.... - Kodama, Tadayuki, Tanaka, Masayuki, Tamura, Takayuki, Yahagi, Hideki, Nagashima, Masahiro, Tanaka, Ichi, Arimoto, Nobuo, Futamase, Toshifumi, Iye, Masanori, Karasawa, Yoshikazu, Kashikawa, Nobunari, Kawasaki, Wataru, Kitayama, Tetsu, Matsuhara, Hideo, Nakata, Fumiaki, Ohashi, Takaya, Ohta, Kouji, Okamoto, Takashi, Okamura, Sadanori, Shimasaku, Kazuhiro, Suto, Yasushi, Tamura, Naoyuki, Umetsu, Keiichi, Yamada, ToruPublications of the Astronomical Society of Japan 57 2 309 - 323 2005年04月 [査読有り][通常論文]
We have started PISCES project; a panoramic imaging and spectroscopic survey of distant clusters on Subaru. It exploits the unique wide-field imaging capability of Suprime-Cam, which provides a 34' × 27' field of view corresponding to a physical area of 16 × 13 Mpc2 at z ∼ 1. We plan to target 15 clusters at 0.4 ≲ z ≲ 1.3. We report on our first results concerning the inner structures and large-scale structures of two distant clusters at z = 0.55 and 0.83 together with the earlier results on a z = 0.41 cluster. The photometric redshift technique has been applied to multi-color data in order to remove most of the foreground/background galaxies so as to isolate the cluster member candidates. We have found large-scale filamentary structures around the clusters, extending out to > 5 Mpc from the cores, as well as complex inner structures. The galaxy distributions in the inner regions of the clusters look similar to the X-ray intensity maps, suggesting that most of the optical structures trace physically bound systems. We also compared the structures of the three clusters with those of model clusters in a numerical simulation (N-body + semi-analytic model) by parameterising the shapes of the iso-density contours of galaxies, and found a broad agreement. Our results that cluster-scale assembly takes place along filaments during hierarchical clustering need to be confirmed spectroscopically in the near future.... - Okamoto, Takashi, Jenkins, Adrian, Eke, Vincent R., Quilis, Vincent, Frenk, Carlos S.Monthly Notices of the Royal Astronomical Society 345 2 429 - 446 2003年10月 [査読有り][通常論文]
We investigate the evolution of angular momentum in smoothed particle hydrodynamic (SPH) simulations of galaxy formation, paying particular attention to artificial numerical effects. We find that a cold gas disc forming in an ambient hot gas halo receives a strong hydrodynamic torque from the hot gas. By splitting the hydrodynamic force into artificial viscosity and pressure gradients, we find that the angular momentum transport is caused not by the artificial viscosity but by the pressure gradients. Using simple test simulations of shear flows, we conclude that the pressure gradient-based viscosity can be divided into two components: one due to the noisiness of SPH and the other due to ram pressure. The former is problematic even with very high resolution, because increasing the resolution does not reduce the noisiness. On the other hand, the ram pressure effect appears only when a cold gas disc or sheet does not contain enough particles. In such a case, holes form in the disc or sheet, and then ram pressure from intra-hole hot gas causes significant deceleration. In simulations of galactic disc formation, star formation usually decreases the number of cold gas particles, and hole formation leads to the fragmentation of the disc. This fragmentation not only induces further angular momentum transport, but also affects star formation in the disc. To circumvent these problems, we modify the SPH algorithm, decoupling the cold gas phases from the hot ones, i.e. inhibiting the hydrodynamic interaction between cold and hot particles. This, a crude modelling of a multiphase fluid in SPH cosmological simulations, leads to the formation of smooth extended cold gas discs and to better numerical convergence. The decoupling is applicable in so far as the self-gravitating gas disc with negligible external pressure is a good approximation for a cold gas disc.... - Suwa, Tamon, Habe, Asao, Yoshikawa, Kohji, Okamoto, TakashiThe Astrophysical Journal 588 1 7 - 17 2003年05月 [査読有り][通常論文]
We investigate how cluster morphology is affected by the cosmological constant in low-density universes. Using high-resolution cosmological N-body/smoothed particle hydrodynamics simulations of flat cold dark matter (ΛCDM Ω0=0.3,λ0=0.7) and open cold dark matter (OCDM; Ω0=0.3,λ0=0) universes, we calculate statistical indicators to quantify the irregularity of the cluster morphologies. We study axial ratios, center shifts, cluster clumpiness, and multipole-moment power ratios as indicators for the simulated clusters at z=0 and 0.5. Some of these indicators are calculated for both the X-ray surface brightness and projected mass distributions. In ΛCDM all these indicators tend to be larger than those in OCDM at z=0. This result is consistent with the analytical prediction of Richstone, Loeb, & Turner, that is, clusters in ΛCDM are formed later than in OCDM and have more substructure at z=0. We perform a Kolmogorov-Smirnov test on each indicator for these two models. We then find that the results for the multipole-moment power ratios and the center shifts for the X-ray surface brightness are under the significance level (5%). Our results also show that these two cosmological models can be distinguished more clearly at z=0 than at z=0.5 by these indicators.... - Okamoto, Takashi, Nagashima, MasahiroThe Astrophysical Journal 587 2 500 - 513 2003年04月 [査読有り][通常論文]
We investigate environmental effects on evolution of bright cluster galaxies (L>L*) in a Λ-dominated cold dark matter universe using a combination of dissipationless N-body simulations and a semianalytic galaxy formation model. The N-body simulations enable us to calculate orbits of galaxies in simulated clusters. Therefore, we can incorporate stripping of cold gas from galactic disks by ram pressure (RP) from the intracluster medium into our model. In this paper we study how ram pressure stripping (RPS) and small starburst induced by a minor merger affect colors, star formation rates (SFRs), and morphologies of cluster galaxies. These processes are new ingredients in our model and have not been studied sufficiently. We find that the RPS is not important for colors and SFRs of galaxies in the cluster core if the star formation timescale is properly chosen, because the star formation is sufficiently suppressed by consumption of the cold gas in the disks. Then observed color and SFR gradients can be reproduced without the RPS. The small starburst triggered by a minor merger hardly affects the SFRs and colors of the galaxies as well. We also examine whether these two processes can resolve the known problem that the hierarchical clustering models based on the major merger-driven bulge formation scenario predict too few galaxies of intermediate bulge-to-total luminosity ratio (B/T) in clusters. When the minor burst is taken into account, the intermediate B/T population is increased, and the observed morphology gradients in clusters are successfully reproduced. Without the minor burst, the RPS cannot increase the intermediate B/T population. On the other hand, when the minor burst is considered, the RPS also plays an important role in formation of the intermediate B/T galaxies. We present redshift evolution of morphological fractions predicted by our models. The predicted number ratios of the intermediate B/T galaxies to the bulge-dominated galaxies show nearly flat or slightly increasing trends with increasing redshift. We conclude that these trends are inevitable when bulges are formed through mergers. We discuss whether our results conflict with observationally suggested NS0/NE evolution in clusters, which is a decreasing function of redshift.... - Okamoto, Takashi, Nagashima, MasahiroThe Astrophysical Journal 547 1 109 - 116 2001年01月 [査読有り][通常論文]
We present a model to investigate the formation and evolution of cluster galaxies using cosmological high-resolution N-body simulations. The N-body simulations are used to construct merging history trees of dark halos. Gas cooling, star formation, supernova feedback, and mergers of galaxies within dark halos are included by using simple prescriptions taken from semianalytic models of galaxy formation. In this paper, we examine the merger-driven bulge formation model and represent the morphology-density relation of cluster galaxies at z=0. We find that this morphological evolution model can explain the distribution of elliptical galaxies in the clusters well and cannot reproduce the distribution of S0 galaxies. This result suggests that the elliptical galaxies are mainly formed by the major mergers, while, in the S0 formation, processes other than major mergers play an important role.... - Okamoto, Takashi, Habe, AsaoPublications of the Astronomical Society of Japan 52 3 457 - 463 2000年06月 [査読有り][通常論文]
Using cosmological N-body simulations of critical (SCDM) and open (Omega = 0.3, OCDM) cold dark matter models, we investigated the evolution of cluster galaxies. Based on our numerical simulation, we constructed merging history trees of the galaxies. By following their merging history, we could show that the major merger fractions of the galaxies in cluster-forming regions is roughly proportional to (1+z)4.5 at low redshifts (z < 2), and has a steep peak at z =~ 2.5 and z =~ 3 in SCDM and OCDM, respectively. We also show that the cluster galaxies are affected by the tidal interaction after the clusters are formed. Because the formation redshift of the cluster in SCDM, z_form = 0.15, is much more recent than that of the cluster in OCDM, z_form = 1.6, the cluster galaxies in SCDM show more rapid evolution by tidal interactions from z = 0.5 than those in OCDM.... - Okamoto, Takashi, Habe, AsaoThe Astrophysical Journal 240 2 57 - 58 1999年05月 [査読有り][通常論文]
We investigate effects of time evolution of a rich cluster of galaxies on its member galactic halos in the standard cold dark matter (SCDM) universe using high resolution N-body simulations. We identify several hundred galactic halos within the virial radius of our simulated cluster. We also find that a large number of halos have been tidally disrupted at z=0. Therefore we improve a method of deriving merging history trees of galaxies taking account of tidally stripped galaxies. The main results are as follows: (1) At high redshift (z~=2), the mass function of the galactic halos that are in the cluster at z=0 is very similar to that obtained in the field region and agrees well with the Press-Schechter mass function. (2) The mass function of cluster galaxies that consist of both galactic halos and tidally stripped galaxies has hardly evolved since z~=2. This mass function at z=0 is well represented by the Press-Schechter mass function at z=2. (3) At high redshift (z>3), in the region that becomes the cluster, the fraction of galaxies that have undergone recent merger is larger than that in the field. After z~3, however, it rapidly decreases and becomes smaller than that in the field. (4) The fraction of strongly stripped galaxies among the cluster galaxies begins to increase from z~=0.5. At z=0, a clear correlation appears between this fraction and the distance from the center of the cluster. (5) Tidally truncated halos have steeper outer profiles than those of the model of Navarro, Frenk, & White....
その他活動・業績
- Takashi Okamoto Proceedings of the International Astronomical Union 14 (S344) 498 -501 2018年08月
- Ryu Makiya, Motohiro Enoki, Tomoaki Ishiyama, Masakazu A.R. Kobayashi, Masahiro Nagashima, Takashi Okamoto, Katsuya Okoshi, Taira Oogi, Hikari Shirakata VizieR Online Data Catalog 2016年10月 [査読無し][通常論文]
The mock galaxy catalog calculated by the nu2-GC model descri the paper.
File light-cone_nu2gc-H2.dat contains the galaxies having B-band apparent magnitude brighter than 28 [AB mag] and located at the redshift range 0<z<20.0, with 1 deg2 aperture.
We also provide "box-type" mock galaxy catalogs, catalognu2gc-Lz**.dat, that contain the simulated galaxies in the 1.123 (Gpc/h)3 size box at redshift z = 0.0, 0.48, 1.01, 1.97, 2.95, 6.97. For each simulated galaxy, redshift, comoving distance, stellar mass, gas mass, black hole mass, star formation rate, metallicity, and magnitude in B-, V-, SDSS-rp and 2MASS-Ks bands are given.
Terms of Use : When you write papers using these catalogs, we would be very grateful if you could refer the following two papers: Ishiyama et al., 2015PASJ...67...61I, Makiya et al., 2016PASJ...68...25M.
(7 data files). - Takashi Okamoto FIRST STARS IV - FROM HAYASHI TO THE FUTURE 1480 403 -405 2012年 [査読有り][通常論文]
Bulges can be classified into classical and pseudobulges; the former are considered to be end products of galactic mergers and the latter to form via secular evolution of galactic disks. Observationally, bulges of disk galaxies are mostly pseudobulges, including the Milky Way's. We here show, by using self-consistent cosmological simulations of galaxy formation, that the formation of pseudobulges of Milky Way-sized disk galaxies has mostly completed before disk formation; thus the main channel of pseudobulge formation is not secular evolution of disks. Our pseudobulges form by rapid gas supply at high-redshift and their progenitors would be observed as high-redshift disks. - Takayuki R. Saitoh, Hiroshi Daisaka, Eiichiro Kokubo, Junichiro Makino, Takashi Okamoto, Kohji Tomisaka, Keiichi Wada, Naoki Yoshida COMPUTATIONAL STAR FORMATION 270 (270) 483 -486 2011年 [査読無し][通常論文]
We studied the formation process of star clusters using high-resolution N-body/smoothed particle hydrodynamics simulations of colliding galaxies. The total number of particles is 1.2 x 10(8) for our high resolution run. The gravitational softening is 5 pc and we allow gas to cool clown to similar to 10 K. During the first encounter of the collision, a giant filament consists of cold and dense gas found between the progenitors by shock compression. A vigorous starburst took place in the filament, resulting in the formation of star clusters. The mass of these star clusters ranges from 10(5-8) M-circle dot. These star clusters formed hierarchically: at first small star clusters formed, and then they merged via gravity, resulting in larger star clusters. - Cambridge University PressComputational Star Formation, Proceedings of the International Astronomical Union, IAU Symposium 270 483 -486 2011年 [査読無し][通常論文]
- American Institute of PhysicsHUNTING FOR THE DARK: THE HIDDEN SIDE OF GALAXY FORMATION. Edited by Victor P. Debattista and Cristina C. Popescu AIP Conference Proceedings, Volume 1240 1240 399 -402 2010年 [査読無し][通常論文]
- Takashi Okamoto AIP Conference Proceedings 1240 399 -402 2010年 [査読無し][通常論文]
We study the formation histories and present-day structure of galactic subsystems formed in a high resolution hydrodynamic simulation of a Milky Way-sized galaxy. The simulated satellites span nearly 4 orders of magnitude in luminosity but have a very similar mass within their inner 600 pc with very little scatter. This is in agreement with the recent measurements for dwarf spheroidal galaxies (dSphs). Interestingly, subhaloes with circular velocity around 20 km/s sometimes host satellites that are as bright as the classical satellites and sometimes they are found as completely dark objects. We find that there is a critical circular velocity at the epoch of reionization, below which haloes fail to host star formation and found as "dark satellites" today. © 2010 American Institute of Physics. - 岡本 崇 天文月報 102 (12) 759 -768 2009年11月20日
- Masahiro Nagashima, Takashi Okamoto PANORAMIC VIEWS OF GALAXY FORMATION AND EVOLUTION, PROCEEDINGS 399 475 -+ 2008年 [査読無し][通常論文]
We present chemical properties of dwarf galaxies in a semi-analytic galaxy formation model. We find a good match with observations of the Local dwarf spheroidals. The supernova feedback is a key to understanding the different metal abundances of the Local Group dwarf spheroidals from those. associated with the Milky Way. - Galaxy FormationAstronomical Society of the PacificNew Horizons in Astronomy: Frank N. Bash Symposium 2007 ASP Conference Series, Vol. 393, proceedings of the conference held 14-16 October 2007 at the University of Texas, Austin, Texas, USA. Edited by Anna Frebel, Jus・・・ 111 -125 2008年 [査読無し][通常論文]
Astronomical Society of the PacificNew Horizons in Astronomy: Frank N. Bash Symposium 2007 ASP Conference Series, Vol. 393, proceedings of the conference held 14-16 October 2007 at the University of Texas, Austin, Texas, USA. Edited by Anna Frebel, Justyn R. Maund, Juntai Shen, and Michael H. Siegel. - Takayuki R. Saitoh, Jin Koda, Takashi Okamoto, Keiichi Wada, Asao Habe MAPPING THE GALAXY AND NEARBY GALAXIES 373 -373 2008年 [査読無し][通常論文]
- Takashi Okamoto Astrophysics and Space Science Proceedings (202419) 367 2008年 [査読無し][通常論文]
- Masahiro Nagashima, Takashi Okamoto PANORAMIC VIEWS OF GALAXY FORMATION AND EVOLUTION, PROCEEDINGS 399 475 -+ 2008年 [査読無し][通常論文]
We present chemical properties of dwarf galaxies in a semi-analytic galaxy formation model. We find a good match with observations of the Local dwarf spheroidals. The supernova feedback is a key to understanding the different metal abundances of the Local Group dwarf spheroidals from those. associated with the Milky Way. - Galaxy FormationAstronomical Society of the PacificNew Horizons in Astronomy: Frank N. Bash Symposium 2007 ASP Conference Series, Vol. 393, proceedings of the conference held 14-16 October 2007 at the University of Texas, Austin, Texas, USA. Edited by Anna Frebel, Jus・・・ 111 -125 2008年 [査読無し][通常論文]
Astronomical Society of the PacificNew Horizons in Astronomy: Frank N. Bash Symposium 2007 ASP Conference Series, Vol. 393, proceedings of the conference held 14-16 October 2007 at the University of Texas, Austin, Texas, USA. Edited by Anna Frebel, Justyn R. Maund, Juntai Shen, and Michael H. Siegel. - Takayuki R. Saitoh, Jin Koda, Takashi Okamoto, Keiichi Wada, Asao Habe MAPPING THE GALAXY AND NEARBY GALAXIES 373 -373 2008年 [査読無し][通常論文]
- Simulations of Coevolving Galaxies and Supermassive Black HolesSpringerMapping the Galaxy and Nearby Galaxies, Astrophysics and Space Science Proceedings 367 2008年 [査読無し][通常論文]
- 斎藤 貴之, 台坂 博, 出田 誠, 岡本 崇, 小久保 英一郎, 和田 桂一, 富阪 幸治, 牧野 淳一郎, 吉田 直紀 日本流体力学会年会講演論文集 2006 286 -286 2006年 [査読無し][通常論文]
In this paper, we introduce Project "Origin of the Milkyway". This project aims at reliable modeling of the formation history of our Galaxy (i.e., the Milkyway), as a typical spiral galaxy, with the mass resolution 2-3 orders of magnitude higher than what has been achieved so far. In order to achieve such state-of-the-art simulations, we construct two beowulf type PC-clusters with GRAPE-6A/7, and we develop a new N-body/SPH code for parallel computing. - M. Nagashima, C. G. Lacey, T. Okamoto, C. M. Baugh, C. S. Frenk, S. Cole ORIGIN OF MATTER AND EVOLUTION OF GALAXIES 847 442 -+ 2006年 [査読無し][通常論文]
We investigate the metal enrichment due to type II and Ia supernovae using semi-analytic models of galaxy formation based on the cold dark matter model of the Universe. - Nick Holliman, Carlton Baugh, Carlos Frenk, Adrian Jenkins, Barbara Froner, Djamel Hassaine, John Helly, Nigel Metcalfe, Takashi Okamoto STEREOSCOPIC DISPLAYS AND VIRTUAL REALITY SYSTEMS XIII 6055 34 -45 2006年 [査読無し][通常論文]
This paper describes our experience making a short stereoscopic movie visualizing the development of structure in the universe during the 13.7 billion years from the Big Bang to the present day. Aimed at a general audience for the Royal Society's 2005 Summer Science Exhibition, the movie illustrates how the latest cosmological theories based on dark matter and dark energy are capable of producing structures as complex as spiral galaxies and allows the viewer to directly compare observations from the real universe with theoretical results. 3D is an inherent feature of the cosmology data sets and stereoscopic visualization provides a natural way to present the images to the viewer, in addition to allowing researchers to visualize these vast, complex data sets. The presentation of the movie used passive, linearly polarized projection onto a 2m wide screen but it was also required to playback on a Sharp RD3D display and in anaglyph projection at venues without dedicated stereoscopic display equipment. Additionally lenticular prints were made from key images in the movie. We discuss the following technical challenges during the stereoscopic production process; 1) Controlling the depth presentation, 2) Editing the stereoscopic sequences, 3) Generating compressed movies in display specific formats. We conclude that the generation of high quality stereoscopic movie content using desktop tools and equipment is feasible. This does require careful quality control and manual intervention but we believe these overheads are worthwhile when presenting inherently 3D data as the result is significantly increased impact and better understanding of complex 3D scenes. - M. Nagashima, C. G. Lacey, T. Okamoto, C. M. Baugh, C. S. Frenk, S. Cole ORIGIN OF MATTER AND EVOLUTION OF GALAXIES 847 442 -+ 2006年 [査読無し][通常論文]
We investigate the metal enrichment due to type II and Ia supernovae using semi-analytic models of galaxy formation based on the cold dark matter model of the Universe. - Nick Holliman, Carlton Baugh, Carlos Frenk, Adrian Jenkins, Barbara Froner, Djamel Hassaine, John Helly, Nigel Metcalfe, Takashi Okamoto STEREOSCOPIC DISPLAYS AND VIRTUAL REALITY SYSTEMS XIII 6055 34 -45 2006年 [査読無し][通常論文]
This paper describes our experience making a short stereoscopic movie visualizing the development of structure in the universe during the 13.7 billion years from the Big Bang to the present day. Aimed at a general audience for the Royal Society's 2005 Summer Science Exhibition, the movie illustrates how the latest cosmological theories based on dark matter and dark energy are capable of producing structures as complex as spiral galaxies and allows the viewer to directly compare observations from the real universe with theoretical results. 3D is an inherent feature of the cosmology data sets and stereoscopic visualization provides a natural way to present the images to the viewer, in addition to allowing researchers to visualize these vast, complex data sets. The presentation of the movie used passive, linearly polarized projection onto a 2m wide screen but it was also required to playback on a Sharp RD3D display and in anaglyph projection at venues without dedicated stereoscopic display equipment. Additionally lenticular prints were made from key images in the movie. We discuss the following technical challenges during the stereoscopic production process; 1) Controlling the depth presentation, 2) Editing the stereoscopic sequences, 3) Generating compressed movies in display specific formats. We conclude that the generation of high quality stereoscopic movie content using desktop tools and equipment is feasible. This does require careful quality control and manual intervention but we believe these overheads are worthwhile when presenting inherently 3D data as the result is significantly increased impact and better understanding of complex 3D scenes. - 斎藤貴之, 幸田仁, 幸田仁, 岡本崇, 岡本崇, 和田桂一, 羽部朝男 日本天文学会年会講演予稿集 2004 2004年
- The roles of ram-pressure stripping and minor mergers in the evolution of galaxiesOutskirts of Galaxy Clusters: Intense Life in the Suburbs. Edited by Antonaldo Diaferio, IAU Colloquium #195 534 -538 2004年 [査読無し][通常論文]
- Structure of Clusters of Galaxies and Open CDM and rHΛ CDMStudies of Galaxies in the Young Universe with New Generation Telescope, Proceedings of Japan-German Seminar 99 -106 2004年 [査読無し][通常論文]
- The roles of ram-pressure stripping and minor mergers in the evolution of galaxiesOutskirts of Galaxy Clusters: Intense Life in the Suburbs. Edited by Antonaldo Diaferio, IAU Colloquium #195 534 -538 2004年 [査読無し][通常論文]
- Structure of Clusters of Galaxies and Open CDM and rHΛ CDMStudies of Galaxies in the Young Universe with New Generation Telescope, Proceedings of Japan-German Seminar 99 -106 2004年 [査読無し][通常論文]
- 斎藤貴之, 幸田仁, 岡本崇, 和田桂一, 羽部朝男 日本天文学会年会講演予稿集 2003 2003年
- T Okamoto, M Nagashima GALAXY EVOLUTION: THEORY AND OBSERVATIONS 17 98 -99 2003年 [査読無し][通常論文]
We investigate environmental effects on morphology evolution of bright cluster galaxies (L > L.) in a Lambda-dominated cold dark matter universe using a combination of N-body simulations and semi-analytic (SA) model. We normalize our model parameters to reproduce the properties of local galaxies, and then we study how ram pressure stripping (RPS) and small starburst triggered by a minor merger (minor burst) affect the morphologies of cluster galaxies. We find that the morphological fraction of galaxies with intermediate bulge-to-total luminosity ratios (B/Ts) cannot be reproduced and the effect of the RPS is negligible unless we consider the minor burst. - T Suwa, A Habe, K Yoshikawa, T Okamoto ASTROPHYSICAL SUPERCOMPUTING USING PARTICLE SIMULATIONS (208) 447 -448 2003年 [査読無し][通常論文]
We calculate some indicators, which are closely related with galaxy clusters' substructure, for each cluster obtained in numerical simulations in OCDM and ACDM, and make a statistical comparison between two models. In ACDM the indicators, multipole moment power ratios and center shifts, are larger than those in OCDM. This result is consistent with the analytical prediction; galaxy clusters' formation epoch in ACDM is later than in OCDM and then clusters in OCDM are more relaxed than those in ACDM. We show that these indicators for X-ray surface brightness are useful tools to distinguish between OCDM and ACDM. - T Saitou, A Habe ASTROPHYSICAL SUPERCOMPUTING USING PARTICLE SIMULATIONS (208) 441 -442 2003年 [査読無し][通常論文]
We study cloud mass function (here after CMF) obtained in a Grape-SPH simulation of model galaxy. The CMF can be fitted by a simple power law, and its power law index is similar to -2.0, which closes to the observational results. - T Okamoto, M Nagashima ASTROPHYSICAL SUPERCOMPUTING USING PARTICLE SIMULATIONS (208) 435 -436 2003年 [査読無し][通常論文]
Using a combination of a cosmological N-body simulation and a semi-analytic galaxy formation model, we investigate environmental effects on cluster galaxies with the ACDM cosmology. We find that stripping of hot gas from galacitc halos as they infall into larger objects sufficiently suppresses star formation in the cluster and successfully reproduces the observed gradients of SFR when we normalize our model to reproduce the observed properties of local galaxies. Consequently, the effect of ram-pressure stripping of cold gas from galactic disks becomes quite small and it is observationally negligible. Therefore, the deficiency of galaxies with intermediate bulge-to-disk luminosity ratios under the hierarchical structure formation cannot be resolved even if we include the ram-pressure stripping in our modeling. - Morphological Evolution of Cluster GalaxiesGalaxy Evolution: Theory & Observations (Eds. Vladimir Avila-Reese, Claudio Firmani, Carlos S. Frenk & Christine Allen) Revista Mexicana de Astronomía y Astrofísica (Serie de Conferencias) 17 98 -99 2003年 [査読無し][通常論文]
- T Suwa, A Habe, K Yoshikawa, T Okamoto ASTROPHYSICAL SUPERCOMPUTING USING PARTICLE SIMULATIONS (208) 447 -448 2003年 [査読無し][通常論文]
We calculate some indicators, which are closely related with galaxy clusters' substructure, for each cluster obtained in numerical simulations in OCDM and ACDM, and make a statistical comparison between two models. In ACDM the indicators, multipole moment power ratios and center shifts, are larger than those in OCDM. This result is consistent with the analytical prediction; galaxy clusters' formation epoch in ACDM is later than in OCDM and then clusters in OCDM are more relaxed than those in ACDM. We show that these indicators for X-ray surface brightness are useful tools to distinguish between OCDM and ACDM. - T Saitou, A Habe ASTROPHYSICAL SUPERCOMPUTING USING PARTICLE SIMULATIONS (208) 441 -442 2003年 [査読無し][通常論文]
We study cloud mass function (here after CMF) obtained in a Grape-SPH simulation of model galaxy. The CMF can be fitted by a simple power law, and its power law index is similar to -2.0, which closes to the observational results. - Environmental Effects on Evolution of Cluster GalaxiesAstronomical Society of the PacificAstrophysical Supercomputing using Particle Simulations, IAU Symposium #208 435 2003年 [査読無し][通常論文]
- Study of Multiphase Interstellar Matter in a Gas Rich Dwarf GalaxyThe Astronomical Society of JapanThe Proceedings of the IAU 8th Asian-Pacific Regional Meeting 2 289 -290 2002年 [査読無し][通常論文]
- Study of Multiphase Interstellar Matter in a Gas Rich Dwarf GalaxyThe Astronomical Society of JapanThe Proceedings of the IAU 8th Asian-Pacific Regional Meeting 2 289 -290 2002年 [査読無し][通常論文]
- T Okamoto, M Nagashima PHYSICS OF GALAXY FORMATION, PROCEEDINGS 222 149 -154 2001年 [査読無し][通常論文]
We present a model to investigate the formation and evolution of cluster galaxies using cosmological high-resolution N-body simulations. The N-body simulations are used to construct merging history trees of galactic dark halos. Gas cooling, star formation, supernova feedback and mergers of galaxies within dark halos are included by using simple prescriptions taken from semi-analytic models of galaxy formation. Using this method, we represent the morphology-density relation for simulated cluster galaxies and the color-magnitude relation of simulated cluster ellipticals under the major merger-driven bulge formation scenario. We find that this morphological evolution model gives a good explanation for the distribution and colors of the cluster ellipticals, while it cannot reproduce the distribution of S0 galaxies. These results suggest that the elliptical galaxies are mainly formed by the major mergers and the processes other than major mergers play important role in the S0 formation. - Properties of Galaxies in Rich ClustersAstronomical Society of the Pacific,The Physics of Galaxy Formation, ASP Conference Proceedings, Vol. 222 222 149 2001年 [査読無し][通常論文]
- Formation and evolution of galactic halos in a cluster of galaxiesElsevierNuclear Physics B Proceedings Supplements 80 2000年 [査読無し][通常論文]
- Formation and evolution of galactic halos in a cluster of galaxiesElsevierNuclear Physics B Proceedings Supplements 80 2000年 [査読無し][通常論文]
- Galaxy Distribution in Clusters of GalaxiesKluwer AcademicProceedings of the 183rd symposium of the International Astronomical Union held in Kyoto, Japan, August 18-22, 1997 262 1999年 [査読無し][通常論文]
- Galaxy Distribution in Clusters of GalaxiesKluwer AcademicProceedings of the 183rd symposium of the International Astronomical Union held in Kyoto, Japan, August 18-22, 1997 262 1999年 [査読無し][通常論文]
受賞
- 2022年03月 日本天文学会 欧文研究報告論文賞
Big Three Dragons: A z = 7.15 Lyman-break galaxy detected in [O III] 88 μm, [C II] 158μm, and dust continuum with ALMA受賞者: Takuya Hashimoto;Akio K. Inoue;Ken Mawatari;Yoichi Tamura;Hiroshi Matsuo;Hisanori Furusawa;Yuichi Harikane;Takatoshi Shibuya;Kirsten K. Knudsen;Kotaro Kohno;Yoshiaki Ono;Erik Zackrisson;Takashi Okamoto;Nobunari Kashikawa;Pascal A. Oesch;Masami Ouchi;Kazuaki Ota;Ikkoh Shimizu;Yoshiaki Taniguchi;Hideki Umehata;Darach Watson
共同研究・競争的資金等の研究課題
- 宇宙構造形成理論から迫るダークマター日本学術振興会：科学研究費助成事業 学術変革領域研究(A)研究期間 : 2020年11月 -2025年03月代表者 : 安藤 真一郎, 樽家 篤史, 岡本 崇, 西道 啓博, 白崎 正人, Dalal Neal
- 銀河団形成領域の高分解能計算と広視野・高分解能観測で解き明かす銀河と銀河団の進化日本学術振興会：科学研究費助成事業 基盤研究(B)研究期間 : 2019年04月 -2022年03月代表者 : 岡本 崇, 諸隈 佳菜, 児玉 忠恭銀河団内に形成される巨大楕円銀河は現在ほとんど星形成活動をしていない．このような大質量銀河の星形成を抑制するには銀河中心に存在する超大質量ブラックホール (SMBH) をエンジンとする活動銀河核からのエネルギー放出 (AGN フィードバック) が必要であると考えられている．2019 年度は，この SMBH の成長と AGN フィードバックを宇宙論的銀河形成シミュレーションで銀河形成と整合的に扱うためのシミュレーションコードを開発した．その際，シミュレーションの分解能に結果が強く依存しないように，SMBH へのガス降着は高分解能シミュレーションの結果にもとづいて実装した． まず，AGN フィードバックよりも超新星爆発が星形成の抑制を担っていると考えられている天の川銀河程度の質量の銀河のシミュレーションをそれぞれ AGN フィードバックをありとなしの 2 種類行った．AGN フィードバックとしては AGN 光度の 2% (降着質量エネルギーの 0.2%）をフィードバックの効率として仮定した． その結果，AGN フィードバックを入れても，母銀河の星形成史はそれがない場合と比較してほとんど変化しなかった．一方，SMBH の成長は AGN フィードバックにより強く抑制されて，最終的な SMBH の質量は AGN フィードバックを考慮したものの方がそうでもないものよりも1桁以上小さかった． このことは，バルジの星質量や速度分散と SMBH 質量の観測的関係を用いて AGN フィードバックの効率を決定できることを意味する．こうして決定した AGN フィードバックを用いて大質量銀河のシミュレーション（そこでは AGN フィードバックにより銀河の星形成が抑制されていると考えられている）を行うことにより，AGN フィードバックに今回考えたものと別のモードが必要かどうかを明らかにできる．
- 星間ガスの内部構造及びダストの分布を考慮した銀河の形成進化の研究日本学術振興会：科学研究費助成事業 特別研究員奨励費研究期間 : 2004年 -2006年代表者 : 岡本 崇自己重力・流体力学用シュミレーションコードに、銀河形成を調べる上で必要となる、ガス冷却、星形成、超新星爆発によるガスの過熱と重元素汚染等の効果を組み入れ、宇宙論的な銀河形成を研究できるようにした。さらに、銀河中心巨大ブラックホールの共進化を調べた。結果は、国際会議「Mapping the Galaxy and Nearby Galaxies]で報告した。現在Monthly Notice of Royal Astronomical Societyに論文を投稿中である。 また、過去に行った宇宙論的な銀河形成シュミレーションを解析することにより、何故天の川銀河の衛星銀河はほぼ同一平面状に分布しているのか、またその平面はなぜ銀河円盤と直行しているのかを明らかにした(Libeskind et al. 2007). 準解析的モデルを用いて、階層的構造形成の元で、銀河内の星の重元素量比と金属量の間の関係が自然に説明できることを示した。また、星形成のタイムスケールの違いにより、銀河円盤と銀河バルジでは、金属量の確率分布及び重元素量比と金属量の間の関係が異なる可能性を指摘した(Nagasima and Okamoto 2006).
- Galaxy Formation
教育活動情報
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- 宇宙物理学１開講年度 : 2021年課程区分 : 修士課程開講学部 : 理学院キーワード : 宇宙物理学，宇宙論、宇宙の構造形成
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