The High Velocity Gas toward Messier 5: Tracing Feedback Flows in the Inner Galaxy

TL;DR

This study detects high velocity, highly-ionized gas near the Galactic disk, with the highest metallicity observed in HVCs, suggesting a Galactic origin possibly linked to nuclear winds or fountain circulation.

Contribution

First evidence of highly-ionized high velocity clouds near the Galactic disk with detailed metallicity measurements indicating a Galactic origin.

Findings

High velocity absorption in multiple ion species at -140 and -110 km/s.

Metallicity of HVCs measured as [O/H]=+0.22, the highest known.

HVCs likely originate within the Galaxy, possibly from nuclear winds or fountain flows.

Abstract

We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS E140M) observations of the post-asymptotic giant branch star ZNG 1 in the globular cluster Messier 5 (l=3.9, b=+47.7; d=7.5 kpc, z=+5.3 kpc). High velocity absorption is seen in C IV, Si IV, O VI, and lower ionization species at LSR velocities of -140 and -110 km/s. We conclude that this gas is not circumstellar on the basis of photoionization models and path length arguments. Thus, the high velocity gas along the ZNG 1 sight line is the first evidence that highly-ionized HVCs can be found near the Galactic disk. We measure the metallicity of these HVCs to be [O/H]=+0.22\pm0.10, the highest of any known HVC. Given the clouds' metallicity and distance constraints, we conclude that these HVCs have a Galactic origin. This sight line probes gas toward the inner Galaxy, and we discuss the possibility that these HVCs may be related to a Galactic nuclear wind or Galactic fountain circulation in the inner regions of the Milky Way.