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

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 intracluster medium (ICM). The re-analysis is made possible by a high-resolution study of the hot ICM 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 ~10 Mpc long filaments at log T(K) ≃ 6, with a baryon overdensity of ~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.