R. Lieu et al., Extreme-ultraviolet images of the clusters of galaxies A2199 and A1795: Clear evidence for a separate and luminous emission component, ASTROPHYS J, 517(2), 1999, pp. L91-L95
Since all of the five clusters of galaxies observed by the Extreme-Ultravio
let Explorer deep survey telescope are found to possess a diffuse EUV-emitt
ing component that is unrelated to the hot intracluster medium (ICM) at X-r
ay temperatures, the question concerning the nature of this new component h
as been a subject of controversy. Here we present results of an EUV and sof
t X-ray spatial analysis of the rich clusters Abell 2199 and 1795. The EUV
emission does not resemble the X-ray morphology of clusters: at the cluster
core, the EUV contours are organized; at larger radii, they are anisotropi
c and therefore are unrelated to the hot ICM. The ratio of EUV to soft X-ra
y intensity rises with respect to cluster radius, to reach values similar t
o 10 times higher than that expected from the hot ICM. The strong EUV exces
s that exists in the absence of soft X-ray excess poses formidable problems
for the nonthermal (inverse Compton) scenario but may readily be explained
as being due to emission lines from a shocked underionized gas.