Detection of Luminous Intracluster Extreme-Ultraviolet Emission from Abell 1795

Abstract

We have observed the rich cluster Abell 1795 for 90,000 s, using the deep survey telescope aboard the Extreme-Ultraviolet Explorer, with the Lex/B (69-190 eV) passband filter. Very luminous emission was detected, with a marked softening of the spectrum toward the limiting sensitivity radius of 10' (~0.67 Mpc, assuming here and thereafter a Hubble constant of 75 km s−1 Mpc−1), where the EUV intensity is 6 times higher than that expected from the low-energy tail of the thermal X-ray emission, and the resulting EUV luminosity (a model-independent quantity) exceeds that of the X-ray band. The consequence of a thermal model interpretation is the existence of a warm (sub-MK) intracluster gas that is dynamical (cooling times ≤10−4 of a Hubble time) and that contains a significant fraction, perhaps all, of the total missing mass of the cluster. Alternative interpretations of the data also invoke assumptions that have significant ramifications in our understanding of clusters.