Atomic Hydrogen Gas in Dark-Matter Minihalos and the Compact High Velocity Clouds

We calculate the coupled hydrostatic and ionization structures of spherically symmetric, pressure-supported gas clouds that are confined by gravitationally dominant dark-matter (DM) mini-halos and by an external bounding pressure provided by a hot medium. We focus on clouds that are photoionized and heated by the present-day background metagalactic field and determine the conditions for the formation of warm (WNM), and multi-phased (CNM/WNM) neutral atomic hydrogen (HI) cores in the DM-dominated clouds. We consider ΛCDM dark-matter halos with cuspy (NFW) and constant density (Burkert) cores. We compute models for a wide range of halo masses, total cloud gas masses, and external bounding pressures. We present models for the pressure-supported HI structures observed in the Local Group dwarf irregular galaxies Leo A and Sag DIG. We find that the hydrogen gas becomes neutral for projected HI column densities exceeding 10 cm. We identify the HI cloud boundaries observed in Leo A and Sag DIG with the ionization fronts, and we derive an upper limit of PHIM/k < ∼ 100 cm −3 K for the ambient pressure of the intergalactic medium in the Local Group. The observed HI gas scale heights in Leo A and Sag DIG imply characteristic DM densities of