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

Abstract

Previous Hubble Space Telescope and FUSE observations have revealed highly ionized high-velocity clouds (HVCs) or more generally low H I column HVCs along extragalactic sight lines over 70%-90% of the sky. The distances of these HVCs have remained largely unknown hindering from distinguishing 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 spectrum of the early-type star HS 1914+7134 (l = 103°, b = +24°) located in the outer region of the Galaxy at d ~= 14.9 kpc. Two HVCs are detected in absorption at v _LSR = -118 and -180 km s^-1 in several species, including C IV, Si IV, Si III, Al II, C II, Si II, O I, but H I 21 cm emission is only seen at -118 km s^-1. Within 17° of HS 1914+7134, we found HVC absorption of low and high ions at similar velocities toward five extragalactic sight lines, suggesting that these HVCs are related. The component at -118 km s^-1 is likely associated with the outer arm of the Milky Way. The highly ionized HVC at -180 km s^-1 is possibly 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 low H I column density HVCs.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract No. NAS5-26555.