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Session 51 - Galaxy Evolution and the Intergalactic Medium.
Display session, Wednesday, June 10
Atlas Ballroom,
We have recently identified several high velocity (< -100 km/s) clouds in the directions of Mrk 509 and PKS 2155-304 that have unusual ionization properties. They exhibit strong CIV absorption with little or no low ion (CII, SiII) absorption or HI 21cm emission. As the closest known analog to the outer diffuse halos of damped Ly-alpha absorbers and the low N(HI) metal line absorption systems seen in the spectra of high redshift quasars, these CIV-HVCs present unique opportunities for studying the conditions within the Milky Way halo and nearby intergalactic gas. We present new GHRS intermediate-resolution measurements of the absorption lines within these CIV-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 high velocity clouds. The CIV-HVCs have ionization properties consistent with photoionization by extragalactic background radiation, though some contribution by collisional ionization within a hot plasma cannot be ruled out. The clouds are probably low density [n(H) 10^4 cm^-3], large [greater than several kiloparsecs], and mostly ionized [n(H)/n(HI) 10^-3] regions located well beyond the neutral gas layer of the Galaxy. The presence of weak HI-HVCs detected through their 21cm emission near both sight lines indicates that the CIV-HVCs trace the extended, ionized low density regions of the HI-HVCs. Several lines of evidence, including very low thermal pressures, favor a location for the CIV-HVCs in the very distant Galactic halo or the Local Group. If the clouds are intergalactic in nature, their metallicities could be [Z/H] -1 or lower, but higher metallicities are favored if the clouds are located in the distant Galactic halo.
