Are Compact High‐Velocity Clouds Extragalactic Objects?

Abstract

Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in the Lo- cal Group model and, at d � 1 Mpc, they have HI volume densities of � 3×10 −4 cm −3 . Clouds with these volume densities and the observed column densities NHI � 10 19 cm −2 will be largely ionized, even if exposed only to the extragalactic ionizing radiation field. Here we examine the implications of this process for models of CHVCs. We have mod- eled 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 sug- gested distances, exposed to an extragalactic ionizing photon fluxi � 10 4 phot cm −2 s −1 . Constant-density cloud models in which the CHVCs are at Local Group distances have total (ionized plus neutral) gas masses � 20 30 times larger than the neutral gas masses, implying that the gas mass alone of the observed population of CHVCs is � 4 × 10 10 M⊙. With a realistic (10:1) dark matter to gas mass ratio, the total mass in such CHVCs is a significant fraction of the dynamical mass of the Local Group, and their line widths would greatly exceed the observedV . Self-consistent models of gas in dark matter halos fare even more poorly; they must lie within approximately 200 kpc of the Galaxy, and (for a given distance) are much more massive than the correspond- ing uniform density models. We also show that exponential neutral hydrogen column density profiles are a natural consequence of an external source of ionizing photons, and argue that these profiles cannot be used to derive model-independent distances to the CHVCs. These results argue strongly that the CHVCs are not cosmological objects, and are instead associated with the Galactic halo. Subject headings: ISM: clouds — ISM: H I — Galaxy: halo — Local Group