Satellite Alignment: I. Distribution of Substructures and Their Dependence On Assembly History From N-Body Simulations

摘要

Observations have shown that the spatial distribution of satellite galaxiesis not random, but aligned with the major axes of central galaxies. Thisalignment is dependent on galaxy properties, such that red satellites are morestrongly aligned than blue satellites. Theoretical work done to interpret thisphenomena has found that it is due to the non-spherical nature of dark matterhalos. However, most studies over-predict the alignment signal under theassumption that the central galaxy shape follows the shape of the host halo. Itis also not clear whether the color dependence of alignment is due to anassembly bias or an evolution effect. In this paper we study these problemsusing a cosmological N-body simulation. Subhalos are used to trace thepositions of satellite galaxies. It is found that the shape of dark matterhalos are mis-aligned at different radii. If the central galaxy shares the sameshape as the inner host halo, then the alignment effect is weaker and agreeswith observational data. However, it predicts almost no dependence of alignmenton the color of satellite galaxies, though the late accreted subhalos showstronger alignment with the outer layer of the host halo than their earlyaccreted counterparts. We find that this is due to the limitation of pureN-body simulations that satellites galaxies without associated subhalos('orphan galaxies') are not resolved. These orphan (mostly red) satellitesoften reside in the inner region of host halos and should follow the shape ofthe host halo in the inner region.