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) minihalos 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 multiphased (CNM/WNM) neutral atomic hydrogen (H I) cores in the DM-dominated clouds. We consider ΛCDM 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 H I structures observed in the Local Group dwarf irregular galaxies Leo A and Sag DIG. We find that the hydrogen gas becomes neutral for projected H I column densities exceeding 1019 cm-2. We identify the H I 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 H I gas scale heights in Leo A and Sag DIG imply characteristic DM densities of 1.2 amu cm-3 (or 0.03 M⊙ pc-3), consistent with the DM densities previously inferred via H I rotation curve studies of dwarf and low surface brightness galaxies. Leo A and Sag DIG obey the scaling correlations that are expected for typical (median) DM halos in a ΛCDM cosmology, provided that the halos contain constant density cores, as suggested by Burkert. We construct explicit "minihalo" models for the multiphased (and low-metallicity) compact high-velocity H I clouds (CHVCs). If the CHVC halos are drawn from the same family of halos that successfully reproduce the dwarf galaxy observations, then the CHVCs must be "circumgalactic" objects, with characteristic distances of 150 kpc. For such systems we find that multiphased behavior occurs for peak WNM H I column densities between 2 × 1019 and 1 × 1020 cm-2, consistent with observations. In contrast, if the CHVCs are "extragalactic" objects with distances ≳750 kpc, then their associated halos must be very "underconcentrated," with characteristic DM densities ≲0.08 cm-3, much smaller than expected for their mass and significantly smaller than observed in the dwarf galaxies. Furthermore, multiphased cores then require higher shielding columns. We favor the circumgalactic hypothesis. If the large population of CHVCs represent "missing low-mass DM satellites" of the Galaxy, then these H I clouds must be pressure confined to keep the gas neutral. For an implied CHVC minihalo scale velocity of vs = 12 km s-1, the confining pressure must exceed ∼50 cm-3 K. A hot (∼2 × 106 K) Galactic corona could provide the required pressure at 150 kpc. Our static minihalo models are able to account for many properties of the CHVCs, including their observed peak H I columns, core sizes, and multiphased behavior. However, important difficulties remain, including the presence in some objects of extended low column density H I wings and Hα emission line fluxes in several CHVCs that are significantly larger than expected.