Environmental Dependence of Dark Matter Halo Growth I: Halo Merger Rates

TL;DR

This study examines how the local environment influences dark matter halo merger rates, revealing that denser regions significantly increase merger frequency across different halo masses and redshifts, with implications for halo growth models.

Contribution

It extends previous work by quantifying environmental effects on halo merger rates and provides a fitting formula incorporating local density and mass, valid up to redshift 2.

Findings

Merger rates are ~2.5 times higher in dense regions than in voids.

Environmental density strongly correlates with the amplitude of the progenitor mass function.

The extended Press-Schechter model should include environmental dependence.

Abstract

In an earlier paper we quantified the mean merger rate of dark matter haloes as a function of redshift z, descendant halo mass M0, and progenitor halo mass ratio xi using the Millennium simulation of the LCDM cosmology. Here we broaden that study and investigate the dependence of the merger rate of haloes on their surrounding environment. A number of local mass overdensity variables, both including and excluding the halo mass itself, are tested as measures of a halo's environment. The simple functional dependence on z, M0, and xi of the merger rate found in our earlier work is largely preserved in different environments, but we find that the overall amplitude of the merger rate has a strong positive correlation with the environmental densities. For galaxy-mass haloes, we find mergers to occur ~2.5 times more frequently in the densest regions than in voids at both z=0 and higher redshifts. Higher-mass haloes show similar trends. We present a fitting form for this environmental dependence that is a function of both mass and local density and is valid out to z=2. The amplitude of the progenitor (or conditional) mass function shows a similarly strong correlation with local overdensity, suggesting that the extended Press-Schechter model for halo growth needs to be modified to incorporate environmental effects.