Origin(s) of the Highly Ionized High-Velocity Clouds Based on Their Distances

TL;DR

This study detects highly ionized high-velocity clouds (HVCs) in the Milky Way using stellar and extragalactic sightlines, providing evidence for a Galactic origin of these clouds through their distances and ionization states.

Contribution

First detection of highly ionized HVCs in the COS spectrum of a Galactic star, establishing their likely Galactic origin and relation to the Milky Way's structure.

Findings

Detection of HVCs at -118 and -180 km/s in multiple ions.

HVC at -118 km/s associated with the Outer Arm.

HVC at -180 km/s plunging onto the Galactic disk.

Abstract

Previous HST and FUSE observations have revealed highly ionized high-velocity clouds (HVCs) or more generally low HI column HVCs along extragalactic sightlines over 70-90% of the sky. The distances of these HVCs have remained largely unknown hampering to distinguish a "Galactic" origin (e.g., outflow, inflow) from a "Local Group" origin (e.g., warm-hot intergalactic medium). We present the first detection of highly ionized HVCs in the Cosmic Origins Spectrograph (COS) spectrum of the early-type star HS1914+7134 (l = 103, b=+24) located in the outer region of the Galaxy at 14.9 kpc. Two HVCs are detected in absorption at v_LSR = -118 and -180 km/s in several species, including CIV, SiIV, SiIII, CII, SiII, OI, but HI 21-cm emission is only seen at -118 \km. Within 17 degrees of HS1914+7134, we found HVC absorption of low and high ions at similar velocities toward 5 extragalactic sight lines, suggesting that these HVCs are related. The component at -118 km/s is likely associated with the Outer Arm of the Milky Way. The highly ionized HVC at -180 km/s is an HVC plunging at high speed onto the thick disk of the Milky Way. This is the second detection of highly ionized HVCs toward Galactic stars, supporting a "Galactic" origin for at least some of these clouds.