2018ApJ...856....5Q

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 that 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 in the outskirts of massive galaxies due to the low densities. The multiphase 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., O VI, O VII, Ne VIII, Mg X, and O VIII). We find that the O VI column density has a narrow range around ≃10¹⁴ cm^–2 for halo masses from M_* ≃ 3 x 10¹⁰ M_☉ to 6 x 10¹² M_☉, which is consistent with some but not all observational studies. For galaxies with halo masses <=3 x 10¹¹ M_☉, photoionization produces most of the O VI, while for more massive galaxies, the O VI is from the medium that is cooling from higher temperatures. Fitting the Galactic (Milky-Way) O VII and O VIII suggests a gaseous halo model where the metallicity is ≃0.55 Z_☉ and the gaseous halo has a maximum temperature of ≃1.9 x 10⁶ K. This gaseous halo model does not close the census of baryonic material within R₂₀₀.