On the Extreme Ultraviolet Emission from Galaxy Clusters

Abstract

An extremely soft X-ray excess throughout galaxy clusters has been claimed as a new feature of these sytems, with important physical implications. We have reexamined this feature in the five clusters for which it has been discussed, using the most recent X-ray absorption cross sections, X-ray data processing techniques, and a consistent set of HI data. For the Virgocluster, we find that the spectrum can be fit with a single-temperature thermal plasma and with an X-ray absorption column that is not significantly different than the Galactic HI column. The result for Abell 1367, Abell 1656 (Coma), Abell 1795, and Abell 2199 is similar in that the difference betweenthe X-ray absorption column and the Galactic HI column is less than 3-sigma for He/H = 0.09, and for He/H = 0.10 only one cluster location leads to a Galactic HI column more than 3-sigma above the X-ray absorption column (Coma, with one location with a 3.6-sigma difference). We conclude that there is no strong evidence for the extremely soft X-ray excess in galaxy clusters.