Missing baryons in shells around galaxy clusters

TL;DR

This paper proposes that a significant portion of missing baryons in galaxy clusters may be hidden in outer shells due to non-equilibrium conditions, supported by theoretical estimates and predictions of EUV emission haloes.

Contribution

It introduces a new hypothesis that missing baryons are concealed in outer shells around clusters where thermal equilibrium is not achieved, supported by mass condition analysis.

Findings

Cluster masses are sufficient to hide baryons in outer shells.

Non-equilibrium conditions prevent baryon detection in outskirts.

Predicted EUV haloes around galaxy clusters.

Abstract

The cluster baryon fraction is estimated from the CMB-scattering leptonic component of the intracluster medium (ICM); however, the observed cluster baryon fraction is less than the cosmic one. Understanding the origin of this discrepancy is necessary for correctly describing the structure of the ICM. We estimate the baryonic mass in the outskirts of galaxy clusters which is difficult to observe because of low electron temperature and density in these regions. The time scale for the electrons and protons to reach equipartition in the outskirts is longer than the cluster age. Since thermal equilibrium is not achieved, a significant fraction of the ICM baryons may be hidden in shells around galaxy clusters. We derive the necessary condition on the cluster mass for the concealment of missing baryons in an outer baryon shell and show that this condition is fulfilled because cluster masses are comparable to the estimated characteristic mass $M=e^4/(m^3_{p} G^2)=1.3x10^{15}$ solar masses. The existence of extreme-ultraviolet emission haloes around galaxy clusters is predicted.