Properties of Dark Matter Halos as a Function of Local Environment Density

TL;DR

This study investigates how dark matter halo properties vary with local environment density, revealing that high-density regions influence halo accretion, shape, spin, and mass loss, with implications for galaxy formation models.

Contribution

It provides a detailed analysis of environmental effects on dark matter halos, highlighting new correlations between local density and halo characteristics across different mass scales.

Findings

High-density environments lead to lower halo spins and higher concentrations.

Halos in dense regions tend to be more spherical and experience earlier mass accretion.

Density distribution in simulations follows an Extreme Value Distribution that broadens over time.

Abstract

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.