The Mass and Absorption Columns of Galactic Gaseous Halos
  The galactic gaseous halo is a gas reservoir for the interstellar medium in
the galaxy disk, supplying materials for star formation. We developed a gaseous
halo model connecting the galaxy disk and the gaseous halo by assuming the star
formation rate on the disk is balanced by the radiative cooling rate of the
gaseous halo, including stellar feedback. In addition to a single-temperature
gaseous halo in collisional ionization equilibrium, we also consider the
photoionization effect and a steady-state cooling model. Photoionization is
important for modifying the ion distribution in low-mass galaxies and outskirts
of massive galaxies due to the low densities. The multi-phase cooling model
dominates the region within the cooling radius, where t_cooling=t_Hubble. Our
model reproduces most of the observed high ionization state ions for a wide
range of galaxy masses (i.e., OVI, OVII, NeVIII, MgX, and OVIII). We find that
the OVI column density has a narrow range around ~10^14 cm^-2 for halo masses
from M_star ~ 3 * 10^10 Msun to 6*10^12 Msun, which is consistent with some but
not all observational studies. For galaxies with halo masses <~ 3 * 10^11 Msun,
photoionization produces most of the OVI, while for more massive galaxies, the
OVI is from the medium that is cooling from higher temperatures. Fitting the
Galactic (Milky-Way) OVII and OVIII suggests a gaseous halo model where the
metallicity is ~0.55 Zsun and the gaseous halo has a maximum temperature of ~
1.9 * 10^6 K. This gaseous halo model does not close the census of baryonic
material within R200.
