A WHIM origin for the soft excess emission in the Coma cluster

TL;DR

This study re-analyzes ROSAT data of the Coma cluster, confirming a soft X-ray excess likely originating from WHIM filaments, supporting the hypothesis that these filaments contain the missing low-redshift baryons.

Contribution

It provides new evidence linking the soft X-ray excess in the Coma cluster to WHIM filaments, supporting their role as the missing baryons in the universe.

Findings

Confirmed strong soft X-ray excess in Coma cluster

Quantitative match with WHIM filament properties in simulations

Supports WHIM filaments as the source of missing baryons

Abstract

This paper provides a new analysis of ROSAT observations of the Coma cluster, to determine the amount of soft X--ray radiation in excess of the contribution from the hot intra--cluster medium. The re--analysis is made possible by a high--resolution study of the hot intra--cluster medium with the XMM-Newton and Planck telescopes out to the cluster's virial radius. The analysis confirms the original findings of a strong excess of soft X--ray radiation, which is likely to be of thermal origin. We find quantitative agreement between the detected soft excess and the physical characteristics of warm--hot intergalactic medium (WHIM) filaments seen in hydrodynamical simulations. We conclude that the most plausible explanation for the soft excess is the presence of $\sim 10$~Mpc--long filaments at $\log T(K)\simeq 6$, with a baryon overdensity $\sim 300$, converging towards the Coma cluster. This interpretation therefore provides support for the identification of the missing low--redshift baryons with WHIM filaments, as predicted by numerical simulations.