Oviand Multicomponent HiAbsorption Associated with a Galaxy Group in the Direction of PG 0953+415: Physical Conditions and Baryonic Content

We report the discovery of an O VI absorption system at z(abs) = 0.14232 in ahigh resolution FUV spectrum of PG0953+415 obtained with the Space TelescopeImaging Spectrograph (STIS). Both lines of the O VI 1032, 1038 doublet andmulticomponent H I Lya absorption are detected, but the N V doublet and thestrong lines of C II and Si III are not apparent. We examine the ionizationmechanism of the O VI absorber and find that while theoretical considerationsfavor collisional ionization, it is difficult to observationally rule outphotoionization. If the absorber is collisionally ionized, it may not be inequilibrium due to the rapid cooling of gas in the appropriate temperaturerange. Non-equilibrium collisionally ionized models are shown to be consistentwith the observations. A WIYN survey of galaxy redshifts near the sight linehas revealed a galaxy at a projected distance of 395 kpc separated by ~130 km/sfrom this absorber, and three additional galaxies are found within 130 km/s ofthis redshift with projected separations ranging from 1.0 Mpc to 3.0 Mpc.Combining the STIS observations of PG0953+415 with previous high S/N GHRSobservations of H1821+643, we derive a large number of O VI absorbers per unitredshift, dN/dz ~20. We use this sample to obtain a first estimate of thecosmological mass density of the O VI systems at z ~ 0. If further observationsconfirm the large dN/dz derived for the O VI systems, then these absorberstrace a significant reservoir of baryonic matter at low redshift.Comment: Accepted for publication in Ap.J., vol. 542 (Oct. 10, 2000