The galaxy cluster outskirts probed by Chandra

TL;DR

This study analyzes Chandra X-ray data of 320 galaxy clusters to understand the properties of the intracluster medium in the outskirts, revealing density profile steepening, deviations from sphericity, and baryon fraction behavior.

Contribution

It provides the largest sample-based analysis of ICM properties in cluster outskirts, constraining gas density, slope, and fraction beyond R200 with implications for cluster physics.

Findings

Density profiles steepen beyond R500 and R200

Clusters deviate from spherical symmetry with filament alignment

Gas fraction reaches cosmic value at R200, but systematics may affect measurements

Abstract

We studied the physical properties of the intracluster medium in the virialization region of a sample of 320 clusters ($0.056 <z< 1.24$, $kT>3$ keV) in the Chandra archive. With the emission measure profiles from this large sample, the typical gas density, gas slope and gas fraction can be constrained out to and beyond $R_{200}$. We observe a steepening of the density profiles beyond $R_{500}$ with $\beta \sim 0.68$ at $R_{500}$ and $\beta \sim 1$ at $R_{200}$ and beyond. By tracking the direction of the cosmic filaments approximately with the ICM eccentricity, we report that galaxy clusters deviate from spherical symmetry, with only small differences between relaxed and disturbed systems. We also did not find evolution of the gas density with redshift, confirming its self-similar evolution. The value of the baryon fraction reaches the cosmic value at $R_{200}$: however, systematics due to non-thermal pressure support and clumpiness might enhance the measured gas fraction, leading to an actual deficit of the baryon budget with respect to the primordial value. This study has important implications for understanding the ICM physics in the outskirts.