Investigation Of Galaxy Formation In Semi-Analytical Model

A cold dark matter(ACDM)model is the standard cosmological model.In this model,dark matter halos are formed in the universe,and merge with each other to form larger halos.Baryons could been attracted by halo gravity,and form galaxy via shock wave heating and cooling.Semi-analytical model is an effective method to make a large amount of galaxy formation event in such cosmological frame.In this work,we use LGalaxies,one kind of semi-analytical model produced by Munich research group which could make good prediction comparing to observation,to investigate the formation of neighbor galaxies:the Milky Way(MW)and M101.Meanwhile,we modified the empirical description about super massive black hole(SMBH)accretion in the model by introduce a new mechanism,which is more physical and make better prediction in quasar formation when comparing with the classical model.Galaxies in the local universe,especially MW,are excellent laboratories to check the ACDM model and baryonic physics,attribute to high accurate observation data in their satellites.One of the most confused question for MW is whether the number of satellites is consistent with which in theory.In recent years,some progresses have claimed that it is rare for MW alike galaxies to contain an Large Magellanic Cloud(LMC,the largest satellite in MW),and LMC should affect the distribution of other satellites in MW.In our work,we find that the halo mass is larger in Milky Way analogs if there is a massive satellite galaxy in the system.We also find that the satellites distribution is more disperse and more asymmetric,with more concentrated on the line connecting the central galaxy and the massive satellite.By tracing the formation history of galaxy,we find that LMC has brought in 14.7-5.7+5.3 satellites with Mr<0 when accretion,among which 4.5±2.5 satellites are still in neighbor of LMC.By comparing our model with the data of Satellites Around Galactic Analogs(SAGA),we find that SAGA has more bright satellites and less faint satellites than our model prediction.There is a large magnitude gap between the third and the fourth satellite in M101,and we investigate the property and formation mechanism of this big gap by using LGalaxies.In our work,we find that although M101 analogs are very rare in the local universe(<1%),their formation is a natural outcome of the CDM model.The halo mass of the galaxy has a good correlation with faint end of the gap,we predict that M101 locate in a dark matter halo with Mvir～4×1011M⊙ by this correlation.The gap in magnitude is mainly due to the mass of the accreted subhalos,not from the stochastic star formation in them.We also find that the gap is not in a stable state,and it will disappear in several Gyr due to mergers of bright satellites with the central galaxy.We developed a new black hole growth model by modified the cold accretion process in L-Galaxies model.In our model,the most important way to feed SMBH is the disk instability induced by disk orientation change.The processes which could change the disk orientation are gas cooling and merge,and gas cooling is more important among them.After rescaling the parameters,the stellar mass function and conditional stellar mass function in our model are consistent with which in H15 model,as well as the data in observation.In our model,we reproduced the black hole-bulge mass relation,the two point cross correlation between galaxy and black hole,and have better quasar luminosity function than H15,comparing with observation.After testing several different seeds in our model,we believe that there should be a large amount of super-Eddington accretion in early universe.