The distribution of ejected subhalos and its implication for halo assembly bias

Using a high-resolution cosmologicalN -body simulation, we identify the ejected population of subhalos, which are halos at redshift z = 0 but were once contained in more massive ‘host’ halos at high redshifts. The fraction of the ejected subhalos in the total halo population of the same mass ranges from 9% to 4% for halo masses from ∼ 10 to ∼ 10 hM⊙. Most of the ejected subhalos are distributed within 4 times the virial radius of their hosts. These ejected subhalos have distinct velocity distribution around their hosts in comparison to normal halos. The number of subhalos ejected from a host of given mass increases with the assembly redshift of the host. Ejected subhalos in general reside in high-density regions, and have a much higher bias parameter than normal halos of the same mass. They also have earlier assembly times, so that they contribute to the assembly bias of dark matter halos seen in cosmological simulations. However, the assembly bias is not dominated by the ejected population, indicating that large-scale environmental effects on normal halos are the main source for the assembly bias.