Galaxy Clusters at the Edge: Temperature, Entropy, and Gas Dynamics at the Virial Radius

TL;DR

This study compares Suzaku X-ray observations of galaxy cluster outskirts with Enzo simulations, finding strong agreement in profiles and revealing non-hydrostatic conditions due to accretion-driven turbulence.

Contribution

First comparison of observed and simulated galaxy cluster outskirts showing agreement in temperature and entropy profiles, and highlighting non-hydrostatic gas dynamics.

Findings

Simulations match observed profiles well

Outer cluster gas is turbulent and not in hydrostatic equilibrium

Adiabatic physics suffices for modeling outer regions

Abstract

Recently, Suzaku has produced temperature and entropy profiles, along with profiles of gas density, gas fraction, and mass, for multiple galaxy clusters out to ~r_200 (~= virial radius). In this paper, we compare these novel X-ray observations with results from N-body + hydrodynamic adaptive mesh refinement cosmological simulations using the Enzo code. There is excellent agreement in the temperature, density, and entropy profiles between a sample of 27 mostly substructure-free massive clusters in the simulated volume and the observed clusters. This supports our previous contention that clusters have "universal" outer temperature profiles. Furthermore, it appears that the simplest adiabatic gas physics used in these Enzo simulations is adequate to model the outer regions of these clusters without other mechanisms (e.g., non-gravitational heating, cooling, magnetic fields, or cosmic rays). However, the outskirts of these clusters are not in hydrostatic equilibrium. There is significant bulk flow and turbulence in the outer intracluster medium created by accretion from filaments. Thus, the gas is not fully supported by thermal pressure. The implications for mass estimation from X-ray data are discussed.