Are Compact High‐Velocity Clouds Extragalactic Objects?

Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in theLocal Group model and would have HI volume densities of order 0.0003/cm^3.Clouds with these volume densities and the observed neutral hydrogen columndensities will be largely ionized, even if exposed only to the extragalacticionizing radiation field. Here we examine the implications of this process formodels of CHVCs. We have modeled the ionization structure of spherical clouds(with and without dark matter halos) for a large range of densities and sizes,appropriate to CHVCs over the range of suggested distances, exposed to theextragalactic ionizing photon flux. Constant-density cloud models in which theCHVCs are at Local Group distances have total (ionized plus neutral) gas massesroughly 20-30 times larger than the neutral gas masses, implying that the gasmass alone of the observed population of CHVCs is about 40 billion solarmasses. With a realistic (10:1) dark matter to gas mass ratio, the total massin such CHVCs is a significant fraction of the dynamical mass of the LocalGroup, and their line widths would exceed the observed FWHM. Models with darkmatter halos fare even more poorly; they must lie within approximately 200 kpcof the Galaxy. We show that exponential neutral hydrogen column densityprofiles are a natural consequence of an external source of ionizing photons,and argue that these profiles cannot be used to derive model-independentdistances to the CHVCs. These results argue strongly that the CHVCs are notcosmological objects, and are instead associated with the Galactic halo.Comment: 30 pages, 14 figures; to appear in The Astrophysical Journa