The Dependence on Environment of Cold Dark Matter Halo Properties

Abstract

A series of high-resolution ΛCDM cosmological N-body simulations are used to study the properties of galaxy-size dark halos as a function of global environment. We analyze halos in three types of environment: (cluster halos and their surroundings), (large regions with density contrasts -0.85), and (halos not contained within larger halos). We find that halos in clusters have a median spin parameter ~1.3 times lower, a minor-to-major axial ratio ~1.2 times lower (more spherical), and a less aligned internal angular momentum than halos in voids and the field. For masses 5 × 1011 h-1 M☉, halos in cluster regions are on average ~30%-40% more concentrated and have ~2 times higher central densities than halos in voids. While for halos in cluster regions the concentration parameters decrease on average with mass with a slope of ~0.1, for halos in voids these concentrations do not seem to change with mass. When comparing only parent halos from the samples, the differences are less pronounced but still significant. We obtain also the maximum circular velocity-mass and rms velocity-mass relations. These relations are shallower and more scattered for halos in clusters than in voids, and for a given circular velocity or rms velocity, the mass is smaller at z = 1 than at z = 0 for all environments. At z = 1, the differences in the halo properties with environment almost disappear, suggesting that the differences were established mainly after z ~ 1. The halos in the cluster regions undergo more dramatic changes than those in the field or the voids. The differences in halo properties with environment are due to (1) the dependence of halo formation time on global environment and (2) local effects such as tidal stripping and the tumultuous histories that halos suffer in high-density regions. We calculate seminumerical models of disk galaxy evolution using halos with the concentrations and spin parameters found for the different environments. For a given disk mass, the galaxy disks have higher surface density, larger maximum circular velocity and secular bulge-to-disk ratio, lower gas fraction, and are redder as one goes from void to cluster environments. Although all these trends agree with observations, the latter tend to show more differences, suggesting that physical ingredients not considered here, such as misalignment of angular momentum, halo triaxiality, merging, ram pressure stripping, harassment, etc., play an important role for galaxy evolution, especially in high-density environments.