The Warm-Hot Disk-Halo Interface Below the Perseus Spiral Arm

TL;DR

This study investigates the disk-halo interface of the Perseus arm in the Milky Way, revealing distinct vertical structures and ionization patterns that suggest turbulent mixing layers and possible galactic fountain activity.

Contribution

First detailed analysis of the Perseus arm's disk-halo interface combining multiple absorption spectra to characterize vertical gas structures and ionization states.

Findings

ext{HI} and ext{SII} decline sharply with height up to 1.5 kpc

High ions ext{SiIV} and ext{CIV} remain constant across heights

Uniform ext{CIV}/ ext{SiIV} ratio indicates turbulent mixing layers

Abstract

The Milky Way's disk-halo interface mediates energy and mass exchange between the interstellar thin disk and the halo. In the first detailed study of the Perseus arm's disk-halo interface, we combine HST/STIS and COS absorption spectra toward 6 stars and 23 AGNs projected behind a narrow section ($95\degree<l<145\degree$, $-46\degree<b<0\degree$), providing a unique dataset that bridges the disk and its extended vertical structure in these directions. We measure \SII, \SiIV, and \CIV\ absorption, along with \HI\ 21 cm emission, at heights $-70$ pc to $-3.3$ kpc from the mid-plane. The arm's southern vertical structure shows distinct height-dependent behaviors: \HI\ and \SII\ column densities sharply decline with height up to 1.5 kpc, then continue declining at a much shallower rate at greater heights. In contrast, high ion (Si IV and C IV) column densities remain relatively constant throughout the entire height range. In the disk-halo interface, where warm neutral medium dominates, \SII\ and the high ions show similar kinematics, and we find a remarkably uniform \CIV/\SiIV\ ratio (\cIVsiIVav$\,=2.5 \pm 0.5$) within $-0.9$ to $-3.25$ kpc. Both the kinematic correspondence and high-ion ratio are consistent with the high ions probing turbulent mixing layers at the interfaces between warm/cool and hot gas phases. The AGN sightlines reveal minimal circumgalactic medium (CGM) contribution in the low-velocity gas at $|v_{\rm LSR}|< 100$ \kms. The extraplanar absorbing gas may trace material ejected from previous Galactic fountain activity.