Ionizing Photons and Extreme-Ultraviolet Excesses in Clusters of Galaxies

Abstract

Observations with the Extreme Ultraviolet Explorer satellite are purported to show extreme-ultraviolet (EUV) and soft X-ray excesses in several clusters of galaxies. If interpreted as thermal emission, this would imply the presence of warm (T ~ 106 K) gas in these clusters with a mass comparable to that of gas at coronal temperatures. If true, this would have profound implications for our understanding of galaxy clusters and the distribution of baryons in the universe. Here we show that because of the large ionizing photon emissivities of gas at such low temperatures, the ionizing photon fluxes seen by disk galaxies in the observed clusters can be very large, resulting in minimum emission measures from neutral gas in such disks as high as 100 cm-6 pc. This result is essentially independent of the mechanism actually responsible for producing the alleged EUV excesses. The predicted emission measures in Abell 1795 (z = 0.063) are about an order of magnitude larger than seen in the Reynolds layer of the Galaxy, providing a straightforward observational test of the reality of the EUV excess. New tunable filter Hα images and Wide Field Planetary Camera images from the Hubble Space Telescope archive do not support the existence of the claimed EUV excess.