O vi and Multicomponent H i Absorption Associated with a Galaxy Group in the Direction of PG 0953+415: Physical Conditions and Baryonic Content

Abstract

We report the discovery of an O VI absorption system at zabs=0.14232in a high-resolution FUV spectrum of PG 0953+415 obtained with theSpace Telescope Imaging Spectrograph (STIS). Both lines of the OVI ??1032, 1038 doublet and multicomponent H I Ly? absorption aredetected, but the N V doublet and the strong lines of C II and SiIII are not apparent. We examine the ionization mechanism of theO VI absorber and find that while theoretical considerations favorcollisional ionization, it is difficult to observationally rule outphotoionization. If the absorber is collisionally ionized, it maynot be in equilibrium due to the rapid cooling of gas in the appropriatetemperature range. Nonequilibrium collisionally ionized models areshown to be consistent with the observations. A WIYN survey of galaxyredshifts near the sight line has revealed a galaxy at a projecteddistance of 395 kpc separated by ~130 km s-1 from this absorber,and three additional galaxies are found within 60 m� and z<0.3 over a totalredshift path ?z of only 0.10. Both of these QSOs were originallyobserved to study the low z Ly? lines and should not be biased infavor of O VI detection. If these sight lines are representative,they imply a large number of O VI absorbers per unit redshift, dN/dz~20.The corresponding value of dN/dz for low z Ly? lines with Wr>50 m�is 102+/-16. We use this sample to obtain a first estimate of thecosmological mass density of the O VI systems at z~0. If furtherobservations confirm the large dN/dz derived for the O VI systems,then these absorbers trace a significant reservoir of baryonic matterat low redshift.