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 ^ { \circ } ) 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 } \sim 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 .