Subhalos are Anisotropically Distributed and Aligned with the Smooth Matter Distribution of Their Host Halos

Abstract

We investigate the distributions of subhalos about their hosts in two suites of zoom-in N-body simulations of halo growth -- one suite focused on Milky Way-Mass halos ($\sim 10^{12} \mathrm{M}_{\odot}$) and another focused on cluster-mass halos ($\sim 10^{15} \mathrm{M}_{\odot}$) in the Symphony simulation suite. We find, in agreement with previous work on this subject, that subhalos are distributed anisotropically about their host halos. In particular, the positions of subhalos lie preferentially near the major axes of their host halos, possibly implying that satellite galaxies will exhibit a similar alignment. Furthermore, we show that in two-dimensional projection subhalos are more likely to be observed near the halo center (where the central galaxy presumably resides) when the host halo is projected nearly along its major axis. This projection effect is significant. Within projected radii of a few percent of the virial radius of the host halo, the fraction of mass in subhalos is $\sim 175\%$ larger for Milky Way mass halos and as much as $\sim 195\%$ larger for cluster halos when projected along the major axis as compared to the average from a random projection. This result has consequences for many applications including the interpretation of gravitational lenses. Finally, we find that the orbital angular momentum vector of subhalos is aligned with the angular momentum vector of their host halo, indicating that a significant component of a halo's angular momentum may be carried in its subhalos. This has consequences for galaxy formation models which use host halo angular momentum as a proxy for galaxy momentum.