Satellite Alignment: I. Distribution of Substructures and Their
  Dependence On Assembly History From N-Body Simulations
  Observations have shown that the spatial distribution of satellite galaxies
is not random, but aligned with the major axes of central galaxies. This
alignment is dependent on galaxy properties, such that red satellites are more
strongly aligned than blue satellites. Theoretical work done to interpret this
phenomena has found that it is due to the non-spherical nature of dark matter
halos. However, most studies over-predict the alignment signal under the
assumption that the central galaxy shape follows the shape of the host halo. It
is also not clear whether the color dependence of alignment is due to an
assembly bias or an evolution effect. In this paper we study these problems
using a cosmological N-body simulation. Subhalos are used to trace the
positions of satellite galaxies. It is found that the shape of dark matter
halos are mis-aligned at different radii. If the central galaxy shares the same
shape as the inner host halo, then the alignment effect is weaker and agrees
with observational data. However, it predicts almost no dependence of alignment
on the color of satellite galaxies, though the late accreted subhalos show
stronger alignment with the outer layer of the host halo than their early
accreted counterparts. We find that this is due to the limitation of pure
N-body simulations that satellites galaxies without associated subhalos
('orphan galaxies') are not resolved. These orphan (mostly red) satellites
often reside in the inner region of host halos and should follow the shape of
the host halo in the inner region.
