Photoionized gas in dark matter minihalos in the galactic halo and local group

We present computations of the metals photoionization structures of pressure-supported gas clouds in gravitationally dominant dark matter minihalos in the extended Galactic halo or Local Group environment. We consider low-metallicity (0.1-0.3 Z_⊙) clouds that are photoionized by the present-day metagalactic radiation field and are also pressure confined by a hot external medium. We study the properties of ACDM Burkert (and also NFW) minihalos with characteristic circular velocities from 12 to 30 km s ^-1 (10⁸-2 × 10⁹ M_⊙). We present results for the volume densities and projected column densities of low-ionization metal atoms and ions, C II, N I, O I, Si II, and S II, produced in the neutral cloud cores, and high ions, C IV, N V, O VI, Si III, Si iv, and S III, produced in the ionized shielding envelopes. We examine the possible relationships between the compact H I high-velocity clouds (CHVCs), dwarf galaxies, and high-velocity ionized C IV absorbers, within the context of photoionized minihalo models for such objects. In pressure-confined (P/k ∼ 50 cm^-3 K) minihalo models for the CHVCs the photoionization states are much lower than in the C IV absorbers. However, for lower bounding pressures ≲1 cm^-3 K, in more massive ∼2 × 10⁹ M _⊙ dwarf galaxy-scale halos, the photoionization states in the outer envelopes resemble those observed in the C IV HVCs. An important exception is O VI, for which our photoionization models underpredict the relative abundances by about an order of magnitude, suggesting that additional processes are at play as concluded by Sembach and coworkers. For the cloud size scales expected in median ACDM Burkert halos, a gas metallicity of 0.3 Z _⊙ is required to produce absorption-line strengths comparable to those in the C IV HVCs. We argue that fully ionized and starless "dark galaxies" could be detectable in the local universe as UV metal line absorbers with ionization states similar to the C IV absorbers.