Examine Galaxy Formation Models In The Local Group And High-redshift Universe

Models of galaxy formation have been successful in reproducing the largescale distribution of galaxies and the properties of galaxies in the universe.Nevertheless,there are still several discrepancies when it comes to explaining observations related to the Local Group and high-redshifts galaxies.This thesis delves into several issues in the nearby and high-redshift universe,where current observations and models exhibit notable discrepancies,aiming to ascertain the causes of these disparities(Chapters 3 and 4)and subsequently refine the galaxy formation model(Chapter 5).The content of this thesis is as follows:The observed properties of satellite galaxies of the MW have shown significant discrepancies from theoretical predictions.This has given rise to challenging issues such as the “missing satellite problem” and “the anisotropic distribution of satellite galaxies”,which pose substantial challenges to both the standard cosmological model and galaxy formation models.Among the solutions to these challenges,the factors related to the Local Group environment,the MW-M31 paired configuration,have long been overlooked.In the first study,we conducted a detailed investigation of this issue using advanced N-body simulations and galaxy formation models such as MS,MS-II,and L-Galaxies.We found that,compared to the scenario without a counterpart to M31’s halo,the abundance of substructures within the MW can exceed by 5%-15% when a halo similar to M31’s is present.The excess of abundance increases with the increasing mass of the substructures.We calculate the tidal tensors of the cosmic web and the anisotropy of the tidal fields surrounding the host halos.We found that the paired halos tend to reside in more isotropic environments.This should be a contributing factor to the excess abundance of substructures within paired halos.Furthermore,we found that the presence of M31 does not significantly affect the thickness in the spatial distribution of satellite galaxies,it does tend to align satellite galaxies along the line connecting M31 and the MW’s dark halo.The research results indicate that when addressing the numerous challenges posed by the properties of satellite galaxies in the MW,the influence of the nearby M31 galaxy cannot be ignored,especially when examining the abundance and spatial distribution of these satellite galaxies.The properties of high-redshift galaxies serve as a potential tool for testing galaxy formation models.In 2018,the two Nature articles reported the existence of massive merging galaxies and dense proto-cluster systems at high redshifts.These observable objects could not be adequately explained by current models.In the second study,we utilized the galaxy catalog from the semi-analytic galaxy model MS-W7 to interpret these observations by examining the merger history of massive galaxies.Firstly,we illustrate the merger history of massive highredshift galaxies that described by the galaxy model.Combining this with existing observational data,we found that the model’s predictions for galaxy merger rates are consistent with observations.Additionally,the early-stage mergers in the model predominantly involved gas-rich mergers,characterized by the presence of a substantial amount of cold gas.We also found that the models are capable of reproducing the high-redshift massive merging galaxies and dense galaxy systems observed in our observations.However,the incidence of these events in the catalog is only 0.07% and 0.3%.The presence of a significant quantity of cold gas in highredshift merging galaxies explains the high star formation rates observed in these galaxies.Furthermore,we found that within the galaxy merger scenarios described by the model,the growth mode of black holes that relies on mergers,could not produce high-redshift supermassive black holes as observed.It requires more careful estimations of the supermassive black hole masses.Otherwise,it could suggest modifications of the modeling of the supermassive black hole growth at high redshifts.The current galaxy formation model cannot reproduce the observed data of the growth of high redshift black holes,which inspires us to further study the co-evolution of black holes and galaxies.We found that the current model has a dependence on the mass resolution of simulations,and in the third work,two solutions were proposed for this.Firstly,we introduced a black hole seed model based on dark matter halo mass to compensate for the lack of mergers in simulations with lower resolutions.We found that,this compensation can account for at least 50% of the differences in the high-mass end of the stellar mass function caused by changes in resolution.Secondly,we investigated the impact of the Eddington limit of black hole accretion on the growth of black holes and the evolution of galaxies.We found that if a few black holes could accrete gas at rates exceeding the Eddington limit at high redshifts can allow for rapid growth of black holes at high redshifts.These two approaches showed substantial potential in alleviating the model’s dependence on resolution of simulations.However,they still cannot completely resolve the problem,which indicates that establishing a comprehensive black hole-galaxy co-evolution model remains a major challenge for galaxy models.