Halo Assembly And Environment

This thesis is dedicated to study the assembly and the effects of environments of dark matter halos with both analytic analysis and numerical simulations.Our study is helpful for understanding the current various halo properties and the formation history and evolution of the galaxies within halos.One of the basic statistical parameters of halos is the bias factor.The simplest anal-yses of halo bias assume that halo mass alone determines halo clustering.However,if the large scale environment is fixed,then halo clustering is almost entirely determined by environment,and is almost completely independent of halo mass.We give an ana-lytic explanation for this phenomenon.Our analysis is useful for studies which use the environmental dependence of clustering to constrain cosmological and galaxy formation models.It also shows why many correlations between galaxy properties and environ-ment are merely consequences of the underlying correlations between halos and their environments,and provides a framework for quantifying such inherited correlations.We have studied how halo intrinsic dynamical properties are linked to their forma-tion processes and how both are correlated with the large scale tidal field within which the halos reside at present.Halo merger trees obtained from cosmological N-body sim-ulations are used to identify infall halos that are about to merge with their hosts.We find that the tangential component of the infall velocity increases significantly with the strength of the local tidal field,but no strong correlation is found for the radial compo-nent.These results can be used to explain how the internal velocity anisotropy and spin of halos depend on environment.The position vectors and velocities of infall halos are aligned with the principal axes of the local tidal field,and the alignment depends on the strength of the tidal field.Opposite accretion patterns are found in weak and strong tidal fields,in the sense that in a weak field the accretion flow is dominated by radial mo-tion within the local structure,while a large tangential component is present in a strong field.These findings can be used to understand the strong alignments we find between the principal axes of the internal velocity ellipsoids of halos and the local tidal field,and their dependence on the strength of the tidal field.They also explain why halo spin increases with the strength of the local tidal field,but only in weak tidal fields does the spin-tidal field alignment follow the prediction of the tidal torque theory.Our results can be used to understand the spins of disk galaxies and velocity structures of elliptical galaxies and their correlations with large-scale structure.We have also studied the accretion histories of accreted subhalos.We find,for the very first time,a bimodal formation redshift distribution in simulations.The bimodal feature is originated from the 'universal' two-phase mass accretion histories of halos.The halos that are young at accretion are those on the fast accretion phase,while old halos at accretion are already on the slow accretion phase.This bimodality is found to depend on the redshift significantly,as the accreted halos at high redshift are almost all young population and the accreted halos at low redshifts are almost all old popula-tion.This feature is independent of the host halo mass yet weakly dependent on the accreted halo mass.The bimodal feature is absent in the merger trees built with ex-tended Press-Schechter formalism,suggesting that further improvements are needed for current analytical models.We discuss how this bimodal feature is related with the galaxy bimodality and its implication for connecting halos with galaxies.