# The Nature of Ionized Gas in the Milky Way Galactic Fountain

## TL;DR

This study investigates the ionization, structure, and mass of gas at the Milky Way's disk-halo interface, revealing large-scale coherent ionized structures and small-scale clumpiness, with implications for galactic gas accretion.

## Contribution

It provides new insights into the spatial scale, ionization states, and metal content of halo gas, highlighting the presence of large coherent structures and small clumpy cloudlets at the disk-halo boundary.

## Key findings

- Ionized gas extends up to 3.4 kpc from the disk.
- Large, coherent structures >1 kpc in size are detected.
- Clumpy, small-scale gas clouds are present on <10 pc scales.

## Abstract

We address the spatial scale, ionization structure, mass and metal content of gas at the Milky Way disk-halo interface detected as absorption in the foreground of seven closely-spaced, high-latitude halo blue horizontal branch stars (BHBs) with heights z = 3 - 14 kpc. We detect transitions that trace multiple ionization states (e.g. CaII, FeII, SiIV, CIV) with column densities that remain constant with height from the disk, indicating that the gas most likely lies within z < 3.4 kpc. The intermediate ionization state gas traced by CIV and SiIV is strongly correlated over the full range of transverse separations probed by our sightlines, indicating large, coherent structures greater than 1 kpc in size. The low ionization state material traced by CaII and FeII does not exhibit a correlation with either N$_{\rm HI}$ or transverse separation, implying cloudlets or clumpiness on scales less than 10 pc. We find that the observed ratio log(N_SiIV/ N_CIV), with a median value of -0.69+/-0.04, is sensitive to the total carbon content of the ionized gas under the assumption of either photoionization or collisional ionization. The only self-consistent solution for photoionized gas requires that Si be depleted onto dust by 0.35 dex relative to the solar Si/C ratio, similar to the level of Si depletion in DLAs and in the Milky Way ISM. The allowed range of values for the areal mass infall rate of warm, ionized gas at the disk-halo interface is 0.0003 < dM_gas / dtdA [M_sun kpc^-2 yr^-] < 0.006. Our data support a physical scenario in which the Milky Way is fed by complex, multiphase processes at its disk-halo interface that involve kpc-scale ionized envelopes or streams containing pc-scale, cool clumps.