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 mini-halos in the extended Galactic halo or Local Group environment. We consider low-metallicity (0.1−0.3 solar) clouds that are photoionized by the present-day metagalactic radiation field, and which are also pressure-confined by a hot external medium. We study the properties of ΛCDM Burkert (and also NFW) mini-halos with characteristic circular velocities from 12 to 30 km s (10 – 2× 10 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 mini-halo models for such objects. In pressure-confined (P/k ∼ 50 cm K) mini-halo models for the CHVCs the photoionization states are much lower than in the C IV absorbers. However, for lower bounding pressures . 1 cm K, in more massive ∼ 2× 10 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 et al. (2000; 2003). For the cloud size-scales expected in median ΛCDM Burkert halos, a gas metallicity of 0.3 solar 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.