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

We calculate the coupled hydrostatic and ionization structures ofpressure-supported gas clouds that are confined by gravitationally dominantdark-matter (DM) mini-halos and by an external bounding pressure provided by ahot medium. We focus on clouds that are photoionized and heated by thepresent-day background metagalactic field and determine the conditions for theformation of warm (WNM), and multi-phased (CNM/WNM) neutral atomic hydrogen(HI) cores in the DM-dominated clouds. We consider LCDM dark-matter halos, andwe compute models for a wide range of halo masses, total cloud gas masses, andexternal bounding pressures. We present models for the pressure-supported HIstructures observed in the Local Group dwarf galaxies Leo A and Sag DIG. Wethen construct minihalo models for the multi-phased (and low-metallicity)compact high-velocity HI clouds (CHVCs). If the CHVCs are drawn from the samefamily of halos that successfully reproduce the dwarf galaxy observations, thenthe CHVCs must be "circumgalactic objects" with characteristic distances of 150kpc. For such systems we find that multi-phased behavior occurs for peak WNM HIcolumn densities between 2e19 and 1e20 cm^-2, consistent with observations. Ifthe large population of CHVCs represent "missing low-mass satellites" of theGalaxy, then these clouds must be pressure-confined to keep the gas neutral.For an implied CHVC minihalo scale velocity of v_s=12 km s^-1, the confiningpressure must exceed ~50 cm^-3 K. A hot (2e6 K) Galactic corona could providethe required pressure at 150 kpc.Comment: 82 pages, 17 figures. To appear in Astrophysical Journal Supplement. (Abridged abstract