Properties of Dark Matter Halos as a Function of Local Environment
  Density
  We study how properties of discrete dark matter halos depend on halo
environment, characterized by the mass density around the halos on scales from
0.5 to 16 $h^{-1}{\rm Mpc}$. We find that low mass halos (those less massive
than the characteristic mass $M_{\rm C}$ of halos collapsing at a given epoch)
in high-density environments have lower accretion rates, lower spins, higher
concentrations, and rounder shapes than halos in median density environments.
Halos in median and low-density environments have similar accretion rates and
concentrations, but halos in low density environments have lower spins and are
more elongated. Halos of a given mass in high-density regions accrete material
earlier than halos of the same mass in lower-density regions. All but the most
massive halos in high-density regions are losing mass (i.e., being stripped) at
low redshifts, which causes artificially lowered NFW scale radii and increased
concentrations. Tidal effects are also responsible for the decreasing spins of
low mass halos in high density regions at low redshifts $z < 1$, by
preferentially removing higher angular momentum material from halos. Halos in
low-density regions have lower than average spins because they lack nearby
halos whose tidal fields can spin them up. We also show that the simulation
density distribution is well fit by an Extreme Value Distribution, and that the
density distribution becomes broader with cosmic time.
