Highly ionized high-velocity clouds: Intergalactic gas in the local group or distant gas in the galactic halo?

In the course of our studies of the gaseous halo surrounding the Milky Way, we have recently identified several high-velocity clouds (HVCs; V-LSR < -100 km s(-1)) in the directions of Mrk 509 and PKS 2155 - 304 that have unusual ionization properties. The clouds exhibit strong C Iv absorption with little or no detectable low ion (C II, Si II) absorption or H I 21 cm emission down to very sensitive levels. As the closest known analog to the outer diffuse halos of damped Ly alpha absorbers and the low H I column density metal-line absorption systems seen in the spectra of high-redshift quasars, these "C Iv HVCs" present unique opportunities for relating the conditions within the Milky Way halo and nearby intergalactic gas to the properties of galactic halos at higher redshift. In this paper we present new Goddard High Resolution Spectrograph (GHRS) intermediate-resolution measurements of the absorption lines within these C Iv HVCs and study the ionization properties of the gas in detail. The present data represent the most complete set of measurements available for studying the ionization conditions within HVCs. The C Iv HVCs have ionization properties consistent with photoionization by extragalactic background radiation, although some contribution by collisional ionization within a hot plasma cannot be ruled out. The clouds are probably low density (n(H) similar to 10(-4) cm(-3)), large (greater than several kiloparsecs). and mostly ionized (n(H I)/n(H) similar to 10(-3)) regions located well beyond the neutral gas layer of the Galaxy. The presence of weak H I HVCs detected through their 21 cm emission near both sight lines indicates that the C Iv HVCs trace the extended, ionized, low-density regions of the H I HVCs. Several lines of evidence, including very low thermal pressures (P/k similar to 2 cm(-3) K), favor a location for the C Iv HVCs in the Local Group or very distant Galactic halo. If the clouds are intergalactic in nature, their metallicities could be [Z/H] similar to -1 or lower, but higher metallicities [Z/H] > -1 are favored if the clouds are located in the distant Galactic halo since the cloud sizes scale inversely with metallicity. We provide a summary of the HVCs detected in absorption at intermediate resolution with the GHRS and the IUE satellite and find that C rv HVCs are detected along three of 10 extragalactic sight lines down to a level of log N(C rv) approximate to 13.3 (3 sigma).