Halo Contraction Effect in Hydrodynamic Simulations of Galaxy Formation

TL;DR

This paper demonstrates through multiple hydrodynamic simulations that baryonic processes cause dark matter halos to contract, increasing their inner density, and introduces a revised analytical model to predict this effect accurately.

Contribution

It provides the first comprehensive simulation-based validation of halo contraction across various galaxy systems and offers a new analytical model for predicting the effect.

Findings

Inner dark matter density increases by a factor of several due to baryon dissipation.

The contraction effect varies among systems and cannot be captured by a simple formula.

A revised analytical model predicts the contracted mass profile with about 10% accuracy.

Abstract

The condensation of gas and stars in the inner regions of dark matter halos leads to a more concentrated dark matter distribution. While this effect is based on simple gravitational physics, the question of its validity in hierarchical galaxy formation has led to an active debate in the literature. We use a collection of several state-of-the-art cosmological hydrodynamic simulations to study the halo contraction effect in systems ranging from dwarf galaxies to clusters of galaxies, at high and low redshift. The simulations are run by different groups with different codes and include hierarchical merging, gas cooling, star formation, and stellar feedback. We show that in all our cases the inner dark matter density increases relative to the matching simulation without baryon dissipation, at least by a factor of several. The strength of the contraction effect varies from system to system and cannot be reduced to a simple prescription. We present a revised analytical model that describes the contracted mass profile to an rms accuracy of about 10%. The model can be used to effectively bracket the response of the dark matter halo to baryon dissipation. The halo contraction effect is real and must be included in modeling of the mass distribution of galaxies and galaxy clusters.