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 HS1914+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 HS1914+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. © 2010. The American Astronomical Society. All rights reserved.