Galactic gas flows from halo to disk : tomography and kinematics at the Milky Way’s disk–halo interface.

We present a novel absorption-line survey using 54 blue horizontal branch stars in the Milky Way halo as background sources for detecting gas flows at the disk–halo interface. Distance measurements to high-latitude (b > 60°) background stars at 3.1–13.4 kpc, combined with unprecedented spatial sampling and spectral resolution, allow us to examine the 3D spatial distribution and kinematics of gas flows near the disk. We detect absorption signatures of extraplanar Ca ii and Na i in Keck HIRES spectra and find that their column densities exhibit no trend with distance to the background sources, indicating that these clouds lie within 3.1 kpc of the disk. We calculate covering fractions of f Ca ii = 63%, f Na i = 26%, and f H i = 52%, consistent with a picture of the circumgalactic medium (CGM) that includes multiphase clouds containing small clumps of cool gas within hotter, more diffuse gas. Our measurements constrain the scale of any substructure within these cool clouds to <0.5 kpc. Ca ii and Na i absorption features exhibit an intermediate-velocity (IV) component inflowing at velocities of −75 km s−1 <v < −25 km s−1 relative to the local standard of rest, consistent with previously studied H i structures in this region. We report the new detection of an inflow velocity gradient Δv z ~ 6–9 km s−1 kpc−1 across the Galactic plane. These findings place constraints on the physical and kinematic properties of CGM gas flows through the disk–halo interface and support a galactic fountain model in which cold gas rains back onto the disk