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M.E. Putman, P. Maloney (CASA, University of Colorado)
The anomalous velocity clouds of neutral hydrogen known as the high-velocity clouds may represent the continuing infall of matter onto the Local Group. The survival of these clouds against photoionization depends on their volume density (and therefore distance) and the strength of the extragalactic ionizing photon flux. If compact high-velocity clouds (CHVCs) are an important reservoir of intergalactic gas, scattered throughout the Local Group, they have mean distances of 650 kpc and volume densities of ~4 X 10-4 cm-3. Clouds with these densities and the observed column densities will be largely ionized, even if exposed only to the extragalactic ionizing radiation field. We have modeled the ionization structure of clouds with a large range of densities and sizes, appropriate to CHVCs over the range of suggested distances, exposed to an extragalactic ionizing photon flux of ~104 photons cm-2 s-1. Models in which the CHVCs are at Local Group distances have total (ionized plus neutral) gas masses ~ 40 times larger than the neutral gas masses, implying that the gas mass alone of the observed population of CHVCs is ~1011 solar masses.