New results from H.E.S.S. observations of galaxy clusters

TL;DR

This paper reports deep H.E.S.S. gamma-ray observations of galaxy cluster Abell 85, finding no significant signal and setting constraints on cosmic ray energy content, which challenges some existing models.

Contribution

It provides the first deep VHE gamma-ray limits on Abell 85, constraining cosmic ray energy fractions in galaxy clusters.

Findings

No significant gamma-ray detection in Abell 85.

Limits on cosmic ray energy fraction are around 8%.

Results challenge some theoretical models of cosmic ray content.

Abstract

Clusters of galaxies are believed to contain a significant population of cosmic rays. From the radio and probably hard X-ray bands it is known that clusters are the spatially most extended emitters of non-thermal radiation in the Universe. Due to their content of cosmic rays, galaxy clusters are also potential sources of VHE (>100 GeV) gamma rays. Recently, the massive, nearby cluster Abell 85 has been observed with the H.E.S.S. experiment in VHE gamma rays with a very deep exposure as part of an ongoing campaign. No significant gamma-ray signal has been found at the position of the cluster. The non-detection of this object with H.E.S.S. constrains the total energy of cosmic rays in this system. For a hard spectral index of the cosmic rays of -2.1 and if the cosmic-ray energy density follows the large scale gas density profile, the limit on the fraction of energy in these non-thermal particles with respect to the total thermal energy of the intra-cluster medium is 8% for this particular cluster. This value is at the lower bounds of model predictions.