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

Abstract

We present FUSE and STIS E140M observations of the post-AGB 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 roughly -140 and -110 km s^-1. 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 +/- 0.10, 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.