Differential Accretion of Ionized Low-velocity Gas at the Milky Way’s Disk–Halo Interface

Abstract We measure line-of-sight velocities of metal absorption and H I emission along 132 QSO sight lines in order to study gas accretion and outflow at the disk–halo interface of the Milky Way. While previous studies have focused on high- and intermediate-velocity clouds and complexes, we examine material at predominantly low velocities relative to the local standard of rest (i.e., all absorbers at ∣ v LSR ∣ < 90 km s −1 , and absorbers at 90 < ∣ v LSR ∣ < 150 km s −1 not associated with any well-defined cloud complexes). We find that gas accretion velocities in the northern Galactic hemisphere are correlated with the ionization potential energy of the multiphase metal ions we include in our analysis; more highly ionized material traced by C iv , Si iv , and N v is moving toward the disk 10−15 km s −1 faster than low ionization state material traced by S ii , C ii * , and Ni ii . We interpret this dependence as potential evidence of warm accreting gas cooling as it reaches the disk–halo interface, causing a pileup of slower-moving cool gas. We find that with the number of available sight lines, kinematic modeling cannot rule out exponential density distributions or layers of gas that sandwich the Galactic disk. Our results paint a picture of a complex, dynamic disk–halo interface in which low-velocity material likely plays an important role in fueling star formation in the Milky Way.