Outskirts of Galaxy Clusters

TL;DR

This paper reviews recent advances in understanding the physics, chemistry, and mass distribution of galaxy cluster outskirts through multi-wavelength observations and discusses future research directions.

Contribution

It provides a comprehensive summary of recent observational and theoretical progress in the largely unexplored outer regions of galaxy clusters.

Findings

Significant progress in X-ray, SZ, and lensing observations of cluster outskirts.

Emerging understanding of physical and chemical processes in these regions.

Identification of technical challenges and potential future improvements.

Abstract

Until recently, only about 10% of the total intracluster gas volume had been studied with high accuracy, leaving a vast region essentially unexplored. This is now changing and a wide area of hot gas physics and chemistry awaits discovery in galaxy cluster outskirts. Also, robust large-scale total mass profiles and maps are within reach. First observational and theoretical results in this emerging field have been achieved in recent years with sometimes surprising findings. Here, we summarize and illustrate the relevant underlying physical and chemical processes and review the recent progress in X-ray, Sunyaev--Zel'dovich, and weak gravitational lensing observations of cluster outskirts, including also brief discussions of technical challenges and possible future improvements.