Internal Cluster Structure

TL;DR

This paper reviews the universal density profiles of galaxy clusters, explores the reasons behind their formation, and discusses observational and theoretical insights into their core structures, including effects of baryonic physics.

Contribution

It provides a comprehensive review of the internal structure of galaxy clusters, highlighting the universality of density profiles and the impact of baryonic physics and observations.

Findings

Density profiles are universal, flat within a scale radius, steepening outward.

Simulations show a concentration-mass-redshift relation that differs from some observations.

Baryonic physics can modify the core structure and observable properties of galaxy clusters.

Abstract

The core structure of galaxy clusters is fundamentally important. Even though self-gravitating systems have no stable equilibrium state due to their negative heat capacity, numerical simulations find density profiles which are universal in the sense that they are fairly flat within a scale radius and gradually steepen farther outward, asymptotically approaching a logarithmic slope of $\approx-3$ near the virial radius. We argue that the reason for the formation of this profile is not satisfactorily understood. The ratio between the virial radius and the scale radius, the so-called concentration, is found in simulations to be closely related to the mass and the redshift and low for cluster-sized haloes, but observed to be substantially higher at least in a subset of observed clusters. Haloes formed from cold dark matter should furthermore be richly substructured. We review theoretical and observational aspects of cluster cores here, discuss modifications by baryonic physics and observables that can provide better insight into the internal structure of clusters.