Ionizing photons and extreme-ultraviolet excesses in clusters of galaxies

Observations with the Extreme Ultraviolet Explorer satellite are purported to show extreme-ultraviolet (EUV) and soft X-ray excesses in several cluste rs of galaxies. If interpreted as thermal emission, this would imply the pr esence of warm (T similar to 10(6) K) gas in these clusters with a mass com parable to that of gas at coronal temperatures. If true, this would have pr ofound implications for our understanding of galaxy clusters and the distri bution of baryons in the universe. Here we show that because of the large i onizing photon emissivities of gas at such low temperatures, the ionizing p hoton fluxes seen by disk galaxies in the observed clusters can be very lar ge, resulting in minimum emission measures from neutral gas in such disks a s high as 100 cm(-6) pc. This result is essentially independent of the mech anism actually responsible for producing the alleged EUV excesses. The pred icted emission measures in Abell 1795 (z = 0.063) are about an order of mag nitude larger than seen in the Reynolds layer of the Galaxy, providing a st raightforward observational test of the reality of the EUV excess. New tuna ble filter H alpha images and Wide Field Planetary Camera images from the H ubble Space Telescope archive do not support the existence of the claimed E UV excess.