京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18791号 / 理博第4049号 / 新制||理||1582(附属図書館) / 31742 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 中村 卓史, 教授 谷森 達, 教授 田中 貴浩 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

We introduce a new suite of radiation- hydrodynamical simulations of galaxy formation and reionization called Aurora. The Aurora simulations make use of a spatially adaptive radiative transfer technique that lets us accurately capture the small- scale structure in the gas at the resolution of the hydrodynamics, in cosmological volumes. In addition to ionizing radiation, Aurora includes galactic winds driven by star formation and the enrichment of the universe with metals synthesized in the stars. Our reference simulation uses 2 x 512(3) dark matter and gas particles in a box of size 25 h(-1) comoving Mpc with a force softening scale of at most 0.28 h(-1) kpc. It is accompanied by simulations in larger and smaller boxes and at higher and lower resolution, employing up to 2 x 1024(3) particles, to investigate numerical convergence. All simulations are calibrated to yield simulated star formation rate functions in close agreement with observational constraints at redshift z = 7 and to achieve reionization at z approximate to 8.3, which is consistent with the observed optical depth to reionization. We focus on the design and calibration of the simulations and present some first results. The median stellar metallicities of low- mass galaxies at z = 6 are consistent with the metallicities of dwarf galaxies in the Local Group, which are believed to have formed most of their stars at high redshifts. After reionization, the mean photoionization rate decreases systematically with increasing resolution. This coincides with a systematic increase in the abundance of neutral hydrogen absorbers in the intergalactic medium.

L’époque de réionisation (EoR) est une phase de grands changements qu’a subit l’Univers dans son premier milliard d’années. Suite à l’apparition des premières étoiles et à l’émission de photons énergétique par ces dernières, l’hydrogène a été réionisé. Cette transition a eu un impact sur la formation des galaxies. J’ai activement participé au développement d’EMMA, un code de simulation numérique ayant pour objectif d’étudier les processus à l’œuvre durant l’EoR. J’ai développé et implémenté un modèle de formation et d’évolution stellaire. Ces travaux ont contribué à la réalisation d’une simulation dédiée à l’étude de l’EoR parmi les plus grosses réalisées à l’heure actuelle. J’ai contribué au développement d’outils dédiés à l’exploration de simulations de ce type. J’ai étudié la façon dont le rayonnement s’échappe des galaxies en fonction des paramètres du modèle stellaire, et montré que les supernovæ peuvent augmenter la fraction de photons libérés. J’ai également étudié la propagation des fronts d’ionisation et montré qu’il était possible de réduire la vitesse de la lumière par trois (et ainsi diminuer le temps de calcul du transfert du rayonnement par 3), tout en conservant des résultats corrects ? / The epoch of reionization (EoR) is a phase of big changes in the first billion years of the Universe history. After the apparition of the first stars and the emission of energetic radiation by thoses ones, the hydrogen was reionized. This transition has an impact on the galaxies formations. I was part of the development team of EMMA, a numerical simulation code who aimed to study the processes happening during the EoR. I developed and implement a stellar formation and evolution model. These works contributed to the realisation of one of the biggest simulation dedicated to the study of the EoR yet. I contribute to the development of a tool dedicated to the exploration of this kind of simulations. I study how the radiation escaped the galaxies as a function of the parameters of the stellar model, and showed that supernovae could increase the ratio of escaping photon. I also studied the ionization fronts propagation and showed that the speed of light could be reduced by a factor 3 (and then divide the computational cost of the radiative transfer by 3), while keeping corrects results .

Les trous noirs supermassifs résident dans les centres de la plupart des galaxies massives et on observe des corrélations entre leurs masses et les propriétés de leurs galaxies hôtes. De plus, on observe des trous noirs de plus d’un milliard de masses solaires quelques centaines de millions d’années seulement après le Big Bang. Ces trous noirs supermassifs présents dans l’univers jeune ne sont que le sommet de l’iceberg de l’ensemble de la population de trous noirs, mais ils mettent en question notre compréhension de la formation et de la croissance des premiers trous noirs. Notre nouvelle méthode améliorant le calcul de la densité de colonne de H2 donne des probabilités pour former des graines massives de trous noirs qui sont plus d’un ordre de grandeur plus élevées que prédit précédemment. Nous trouvons que CR7 pourrait être le premier candidat à héberger un tel trou noir formé par effondrement direct et nous démentons l’existence initialement revendiquée d’une population stellaire massive primordial dans CR7. Nous calculons la densité des taux de fusion des trous noirs binaires des premières étoiles et leurs taux de détection avec aLIGO. Notre modèle démontre que les détections des ondes gravitationnelles à venir au cours des prochaines décennies permettront d’imposer des contraintes plus strictes sur les propriétés des premières étoiles et donc sur les scénarios de formation des premiers trous noirs. Nous développons un modèle analytique en 2D de la rétroaction des noyaux actifs de galaxie pour démontrer qu’un profil de disque plus réaliste réduit la quantité de gaz qui est éjectée du halo par rapport aux modèles 1D existants. La rétroaction empêche l’accretion de gaz sur le trou noir central pendant seulement ∼1 million d’année environ, ce qui permet une accretion de gaz presque continue dans le plan du disque. Avec cette thèse, je contribue à une meilleure compréhension de la formation et la croissance des premiers trous noirs supermassifs. / Supermassive black holes reside in the centres of most massive galaxies and we observe correlations between their mass and properties of the host galaxies. Besides this correlation between a galaxy and its central black hole (BH), we see BHs more massive than one billion solar masses already a few hundred million years after the Big Bang. These supermassive BHs at high redshift are just the tip of the iceberg of the entire BH population, but they challenge our understanding of the formation and growth of the first BHs. Our improved method to calculate H2 self-shielding yields probabilities to form massive seed BHs that are more than one order of magnitude higher, than previously expected. We find that CR7 might be the first candidate to host such a direct collapse BH and we disprove the initially claimed existence of a massive metal-free stellar population in CR7. We calculate the merger rate density of binary BHs from the first stars and their detection rates with aLIGO. Our model demonstrates that upcoming detections of gravitational waves in the next decades will allow to put tighter constraints on the properties of the first stars and therefore on formation scenarios of the first BHs. We develop a 2D analytical model of active galactic nuclei-driven outflows to demonstrate that a more realistic disc profile reduces the amount of gas that is ejected out of the halo, compared to existing 1D models. The outflow prevents gas accretion on to the central BH for only about ∼1Myr, which permits almost continuous gas inflow in the disc plane. With this thesis, I contribute to a better understanding of the formation and growth of the first supermassive BHs.

We calculate the angular power spectrum of the cosmic microwave background temperature fluctuations induced by the kinetic Sunyaev-Zel'dovich (kSZ) effect from the epoch of reionization (EOR). We use detailed N-body+radiative-transfer simulations to follow inhomogeneous reionization of the intergalactic medium. For the first time, we take into account the "self-regulation" of reionization: star formation in low-mass dwarf galaxies or minihalos is suppressed if these halos form in the regions that were already ionized or Lyman-Werner dissociated. Some previous work suggested that the amplitude of the kSZ power spectrum from the EOR can be described by a two-parameter family: the epoch of half-ionization and the duration of reionization. However, we argue that this picture applies only to simple forms of the reionization history which are roughly symmetric about the half-ionization epoch. In self-regulated reionization, the universe begins to be ionized early, maintains a low level of ionization for an extended period, and then finishes reionization as soon as high-mass atomically cooling halos dominate. While inclusion of self-regulation affects the amplitude of the kSZ power spectrum only modestly (~10%), it can change the duration of reionization by a factor of more than two. We conclude that the simple two-parameter family does not capture the effect of a physical, yet complex, reionization history caused by self-regulation. When added to the post-reionization kSZ contribution, our prediction for the total kSZ power spectrum is below the current upper bound from the South Pole Telescope. Therefore, the current upper bound on the kSZ effect from the EOR is consistent with our understanding of the physics of reionization. / text

Des trous noirs supermassifs (TNs) de plusieurs millions de masses solaires occupent le centre de la plupart des galaxies proches. La découverte du TN Sagittarius A* au centre de notre galaxie, La Voie lactée, l'a confirmé. Pour autant, certaines galaxies semblent dépourvues de TNs (par exemple NGC205, M33), ou alors ne posséder un TN que de quelques milliers de masses solaires. D'autre part, des TNs dans leur forme la plus lumineuse, appelés quasars, dont la luminosité est plus importante que des centaines de fois celle d'une galaxie toute entière, ont été observés à très grand décalage spectral, lorsque l'Univers n'était alors âgé que d'un milliard d'années. Les modèles de formation des TNs doivent expliquer à la fois l'existence des TNs de faibles masses observés aujourd'hui dans les galaxies de faibles masses, mais aussi leur prodigieux homologues quasars dans l'Univers jeune. La formation des TNs pose encore de nos jours de nombreuses questions: comment se forment les TNs au début de l'histoire de l'Univers? Quelle est leur masse initiale? Quelle est la masse minimale d'une galaxie pour posséder un TN? Pour répondre à ces questions et pour étudier la formation des TNs dans le contexte de l'évolution des galaxies, nous avons utilisé des simulations hydrodynamiques cosmologiques, qui offrent l'avantage de suivre l'évolution temporelle de nombreux processus comme la formation stellaire, l'enrichissement en métaux, les mécanismes de rétroactions des TNs et des supernovae. J'ai particulièrement dirigé mes recherches sur les trois principaux modèles de formation des TNs à partir du reliquat des premières étoiles, d'amas d'étoiles, ou encore par effondrement direct. / Supermassive black holes (BHs) harboured in the center of galaxies have been confirmed with the discovery of Sagittarius A* in the center of our galaxy, the Milky Way. Recent surveys indicate that BHs of millions of solar masses are common in most local galaxies, but also that some local galaxies could be lacking BHs (e.g. NGC205, M33), or at least hosting low-mass BHs of few thousands solar masses. Conversely, massive BHs under their most luminous form are called quasars, and their luminosity can be up to hundred times the luminosity of an entire galaxy. We observe these quasars in the very early Universe, less than a billion years after the Big Bang. BH formation models therefore need to explain both the low-mass BHs that are observed in low-mass galaxies today, but also the prodigious quasars we see in the early Universe.BH formation is still puzzling today, and many questions need to be addressed: How are BHs created in the early Universe? What is their initial mass? How many BHs grow efficiently? What is the occurrence of BH formation in high redshift galaxies? What is the minimum galaxy mass to host a BH? We have used cosmological hydrodynamical simulations to capture BH formation in the context of galaxy formation and evolution. Simulations offer the advantage of following in time the evolution of galaxies, and the processes related to them, such as star formation, metal enrichment, feedback of supernovae and BHs. We have particularly focused our studies on the three main BH formation models: Pop III remnant, stellar cluster, and direct collapse models.

Large spectroscopic surveys are trailblazing endeavours in the study of stellar archaeology and near eld cosmology. Access to homogeneous databases of thousands of stellar spectra allow for a detailed and statistically satisfying look into the chemical abundance distribution of our Galaxy and its surrounding satellites, ultimately working towards a better understanding of galactic chemical evolution. This thesis presents the work of three new studies at the current frontier of stellar archaeology. Through the rst look at carbon-enhanced metal-poor (CEMP) stars using H-band spectra, six new CEMP stars and another seven likely candidates were found within the APOGEE database following Data Release 12. These stars have chemical compositions typical of metal-poor halo stars, however the alpha-abundances of two stars indicate possible origins in an accreted dwarf galaxy. A lack of heavy element spectral lines impedes further sub-classi cation of these CEMP stars, however, based on radial velocity scatter, we predict most are not CEMP-s stars which are typically found in binary systems. This preliminary investigation warrants optical observations to con rm the stellar parameters and low metallicities of these stars, to determine the heavy-element abundance ratios and improve the precision in the derived abundances, and to examine their CEMP sub-classi cations. Additionally, the rst results for the spectroscopic follow up to the Pristine survey are presented. Using a sample of 149 stars, a success rate of 70% for finding stars with [Fe/H]<-2.5 and 22% for finding stars with [Fe/H]<-3.0 is reported, significantly higher than other surveys that typically report success rates of 3-4% for recovering stars with [Fe/H]<-3.0. Finally, the new spectral analysis tool StarNet is introduced. A deep neural network architecture is used to examine both synthetic stellar spectra and SDSS-III APOGEE spectral data and can produce the stellar parameters of temperature, gravity, and metallicity with similar or better precision as the APOGEE pipeline values when trained directly with the APOGEE spectra. StarNet is capable of being trained on synthetic data as well, and is able to reproduce the stellar parameters for both synthetic and APOGEE spectra, including low signal-to-noise spectra, with similar precision to training on the APOGEE spectra itself. The residuals between StarNet predictions and APOGEE DR13 parameters are similar to or better than the di erences between the APOGEE DR13 results and optical high resolution spectral analyses for a subset of benchmark stars. While developed using the APOGEE spectral database (real spectra and corresponding ASSET synthetic data with similar normalization functions), StarNet should be applicable to other large spectroscopic surveys like Pristine. / Graduate

In the standard model of cosmology (ΛCDM) the first stars, star clusters, and galaxies are expected to have formed in short bursts of star formation in low-mass dark matter halos at high redshifts (<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?z%5C,%20%5Csim%20%5C,6-10" data-classname="equation_inline" />). Up to this point, attempts to predict the properties and abundances of these luminous objects have made use of numerically expensive cosmological simulations. On top of being numerically expensive, these simulations often lack the required sub-parsec resolution needed to resolve the formation of compact star clusters and/or neglect possibly dominant stellar feedback processes. Motivated by this, I introduce Anaxagoras, as far as I know the most detailed analytical ab initio model of starbursts in low-mass halos to date. The model incorporates sub-models for gas cooling (including a new determination of the H2-cooling threshold in minihalos), central gas accretion and disk formation (using a new selfsimilarsolution), stellar feedback from radiation pressure (direct stellar radiation, Lyman-<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Calpha" data-classname="equation_inline" data-title="" /> scattering in H I, and multiple scattering of IR photons by dust), stellar winds, expanding HII regions, and (crudely) supernovae. The resulting star formation efficiency is used to predict the fraction of stars that remain gravitationally bound in a cluster following gas expulsion, andwhat fraction escape the central region of the halo, yet remain bound by the dark matter halo. I apply Anaxagoras to study star formation at <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?z%5C,%20%3E%20%5C,6" data-classname="equation_inline" /> in satellite halos of the Milky Way using a halo merger tree code, as well as Population III (Pop III) star formation in minihalos. For the Milky Way setup, hundreds of galaxies are predicted to form with luminosities (<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?L_%7B%5Crm%20V%7D%20%5C,%3C%5C,%20%5Crm%7Bfew%7D%5C,%20%5Ctimes%20%5C,%2010%5E4%20%5C:%20%5Crm%7BL%7D_%7B%5Codot%7D" data-classname="equation_inline" />), half-mass radii (<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Csim%2010-200%5C:%5Crm%20pc" data-classname="equation_inline" />), mass-to-light ratios (<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?M/L_%7B%5Crm%20V%7D%20%5Csim%20100%20-%20%5Crm%7Bfew%7D%20%5C,%5Ctimes%5C,%2010%5E3%20%5C:%5Crm%7BM%7D_%7B%5Codot%7D/%5Crm%7BL%7D_%7B%5Codot%7D" data-classname="equation" />), and ages (<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?13.18%5E%7B+0.29%7D_%7B-0.31%7D%5C:%5Crm%20Gyrs" data-classname="equation_inline" data-title="" />) in good agreement with the observed local population of Ultra-Faint Dwarfs. This shows that ΛCDM is able to explain the properties ofthe faintest dwarf galaxies without fine-tuning. Furthermore, at least ~ 40 compact (initial half-mass radii <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Csim%200.1-5%5C;%5Crm%7Bpc%7D" data-classname="equation_inline" />), old (<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?13.27%5E%7B+0.21%7D_%7B-0.39%7D%5C:%5Crm%20Gyrs" data-classname="equation_inline" />) globular cluster (GC) candidates with initial stellar masses <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?10%5E5%20-%2010%5E6%20%5C:%20%5Crm%7BM%7D_%7B%5Codot%7D" data-classname="equation_inline" />  are predicted to form at the center of low-mass halos, and could survive to the present-day and explain at least a fraction of the observed metal-poor GCs. Their properties are consistent with recent candidates for GCs residing in dark matter halos. Thus, Anaxagoras lends support to the viability of the scenario of GC formation in minihalos. Finally, the formation of Population III (Pop III) stars in minihalos is studied, with the conclusion that if Pop III stars are not overly massive (<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?25%5C:%5Crm%7BM%7D_%7B%5Codot%7D" data-classname="equation_inline" />) between ~ 1 − 30 stars could form per minihalo at <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?z%5C,%20%3E%20%5C,20" data-classname="equation_inline" />, with the number increasing to ~ 10 − 500 stars per minihalo at <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?z%5C,%3C%5C,15" data-classname="equation_inline" /> as Lyman-Werner feedback delay star formation until halos reach larger masses. In the case where Pop III stars are more massive (<img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?140%5C:%5Crm%7BM%7D_%7B%5Codot%7D" data-classname="equation_inline" data-title="" />) most minihalos form just a single star. Due to self-shielding of H2 in minihalos, I find that the cosmological Lyman-Werner background is insufficient to produce Pop III galaxies in atomic-cooling halos, with the implication that the number of massive Pop III galaxies/star clusters in the early Universe has been greatly overestimated in the literature that ignores self-shielding.