# Galactic Gas Flows from Halo to Disk: Tomography and Kinematics at the Milky Way's Disk-Halo Interface

## TL;DR

This study uses a novel absorption line survey of background stars to map and analyze gas flows at the Milky Way's disk-halo interface, revealing inflow velocities and cloud structures supporting a galactic fountain model.

## Contribution

It introduces a new method using background stars to study the 3D distribution and kinematics of gas flows at the disk-halo interface of the Milky Way.

## Key findings

- Detected extraplanar CaII and NaI indicating clouds within 3.1 kpc of the disk.
- Measured inflow velocity gradient of 6-9 km/s/kpc across the Galactic plane.
- Constrained the substructure scale of cool clouds to less than 0.5 kpc.

## Abstract

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.