Highly Ionized High-Velocity Clouds: Hot Intergalactic Medium or Galactic Halo?

• Published in: The Astrophysical journal Vol. 623; no. 1; pp. 196 - 212
• Main Authors: Collins, Joseph A, Shull, J. Michael, Giroux, Mark L
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 10.04.2005; University of Chicago Press
• Subjects: Galactic halos; Quasars; Galaxy halo; Galaxy: halo; Intergalactic matter; Absorption line; High velocity cloud; quasars: absorption lines; ISM: abundances; Abundance; ISM: clouds; Milky Way
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/428566

We use spectroscopic data from the Hubble Space Telescope (HST) and Far Ultraviolet Spectroscopic Explorer (FUSE) to study the wide range of ionization states of the "highly ionized high-velocity clouds" (HVCs). Studied extensively in O VI absorption, these clouds are usually assumed to be infalling gas in the Galactic halo at distances less than 50 kpc. An alternative model attributes the O VI (and O VII X-ray absorption) to cosmological structures of low-density, shock-heated intergalactic gas, distributed over 1-3 Mpc surrounding the Milky Way. The latter interpretation is unlikely, owing to the enormous required mass of gas (4 x 10 super(12) M sub( )). Our detection, in 9 of 12 sight lines, of low-ionization stages (C II/III/IV; Si II/III/IV) at similar high velocities as O VI requires gas densities far above that (n sub(H) - 5 x 10 super(-6) cm super(-3)) associated with the warm-hot intergalactic medium (WHIM). These HVCs are probably cooling, multiphase gas in the Galactic halo, bow shocks, and interfaces between clouds falling through a hot, rotating gaseous halo. The velocity segregation of these HVCs in Galactic coordinates is consistent with a pattern in which infalling clouds reflect the sense of Galactic rotation, with peculiar velocities superposed.