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 z_abs=0.14232 in a high-resolution FUV spectrum of PG 0953+415 obtained with the Space Telescope Imaging Spectrograph (STIS). Both lines of the O VI λλ1032, 1038 doublet and multicomponent H I Lyα absorption are detected, but the N V doublet and the strong lines of C II and Si III are not apparent. We examine the ionization mechanism of the O VI absorber and find that while theoretical considerations favor collisional ionization, it is difficult to observationally rule out photoionization. If the absorber is collisionally ionized, it may not be in equilibrium due to the rapid cooling of gas in the appropriate temperature range. Nonequilibrium collisionally ionized models are shown to be consistent with the observations. A WIYN survey of galaxy redshifts near the sight line has revealed a galaxy at a projected distance of 395 kpc separated by ~130 km s^-1 from this absorber, and three additional galaxies are found within <~130 km s^-1 of this redshift with projected separations ranging from 1.0 Mpc to 3.0 Mpc. All of these galaxies are luminous (0.6-4.0 L_*), and two of them show the [O II] λ3727 emission line indicative of active star formation. The galaxies with [O II] emission are probably normal spirals. Combining the STIS observations of PG 0953+415 with previous high signal-to-noise observations of H1821+643 with the Goddard High Resolution Spectrograph (GHRS), we find two O VI systems with W_r>60 mÅ and z<0.3 over a total redshift path Δz of only 0.10. Both of these QSOs were originally observed to study the low z Lyα lines and should not be biased in favor 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 Lyα lines with W_r>50 mÅ is 102+/-16. We use this sample to obtain a first estimate of the cosmological mass density of the O VI systems at z~0. If further observations confirm the large dN/dz derived for the O VI systems, then these absorbers trace a significant reservoir of baryonic matter at low redshift.

Based on observations obtained with the WIYN Observatory, which is a joint facility of the University of Wisconsin, Indiana University, Yale University, and the National Optical Astronomy Observatories.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.