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 (BHBs) in the Milky Way halo as background sources for detecting gas
flows at the disk-halo interface. Distance measurements to high-latitude ($b$ >
60{\deg}) background stars at 3.1-13.4 kpc, combined with unprecedented spatial
sampling and spectral resolution, allow us to examine the 3-dimensional spatial
distribution and kinematics of gas flows near the disk. We detect absorption
signatures of extraplanar CaII and NaI 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 $f_{CaII}$ = 63%, $f_{NaI}$ = 26%, and $f_{HI}$
= 52%, consistent with a picture of the CGM that includes multi-phase 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. CaII and NaI absorption features exhibit an intermediate-velocity
(IV) component inflowing at velocities of -75 km/s < v < -25 km/s relative to
the local standard of rest, consistent with previously-studied HI structures in
this region. We report the new detection of an inflow velocity gradient
${\Delta}v_z$ ~ 6-9 km/s/kpc 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.
