The Nature of Ionized Gas in the Milky Way Galactic Fountain

Abstract

We address the spatial scale, ionization structure, mass, and metal content of gas at the Milky Way disk–halo interface detected as absorption in the foreground of seven closely spaced, high-latitude halo blue horizontal branch stars with heights z  = 3–14 kpc. We detect transitions that trace multiple ionization states (e.g., Ca ii , Fe ii , Si iv , C iv ) with column densities that remain constant with height from the disk, indicating that the gas most likely lies within z  < 3.4 kpc. The intermediate ionization state gas traced by C iv and Si iv is strongly correlated over the full range of transverse separations probed by our sight lines, indicating large, coherent structures greater than 1 kpc in size. The low ionization state material traced by Ca ii and Fe ii does not exhibit a correlation with either N H i or transverse separation, implying cloudlets or clumpiness on scales less than 10 pc. We find that the observed ratio log( N Si iv / N C iv ), with a median value of −0.69 ± 0.04, is sensitive to the total carbon content of the ionized gas under the assumption of either photoionization or collisional ionization. The only self-consistent solution for photoionized gas requires that Si be depleted onto dust by 0.35 dex relative to the solar Si/C ratio, similar to the level of Si depletion in DLAs and in the Milky Way interstellar medium. The allowed range of values for the areal mass infall rate of warm, ionized gas at the disk−halo interface is 0.0003 <0.006. Our data support a physical scenario in which the Milky Way is fed by complex, multiphase processes at its disk−halo interface that involve kiloparsec-scale ionized envelopes or streams containing parsec-scale, cool clumps.