Extragalactic diffuse neutral hydrogen clouds: Probes of a hot intergalactic medium

Extragalactic diffuse neutral hydrogen clouds constitute a probe of the intergalactic gas by both thermal and dynamical interaction with the surrounding medium. Clouds recently discovered by Mathewson et al. in the Sculptor group place stringent limits on the density and temperature of any intergalactic gas unless the clouds are very highly clumped. The existence of such clouds is probably inconsistent with a thermal bremsstrahlung interpretation of the 30 keV feature in the spectrum of the isotropic X-ray background. We suggest future high-resolution 21 cm studies of the clouds which should be capable of determining whether a hot dense intergalactic medium does indeed exist