Observations with the Extreme Ultraviolet Explorer satellite are purported to show extreme ultraviolet ( EUV ) and soft X-ray excesses in several clusters of galaxies ( Bonamente , Lieu & Mittaz 2001 ) .
If interpreted as thermal emission , this would imply the presence of warm ( T \sim 10 ^ { 6 } 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 H \alpha images and WFPC images from the Hubble Space Telescope archive do not support the existence of the claimed EUV excess .