A Space Telescope Imaging Spectrograph Survey for O VI Absorption Systems at 0.12 < z lesssim 0.5. II. Physical Conditions of the Ionized Gas

We present a complete catalog of 27 O VI absorbers at low redshift (0.12 < z < 0.5) from a blind survey of 16 QSO echelle spectra in the Hubble Space Telescoep Space Telescope Imaging Spectrograph data archive. These absorbers are identified based only on matching line profiles and the expected doublet ratio between the λλ1031, 1037 transitions. Subsequent searches are carried out to identify their associated transitions. Here we present all relevant absorption properties. By considering absorption components of different species that are well aligned in velocity space, we derive gas temperatures and nonthermal broadening values, b_nt. We show that in all 16 cases considered the observed line width is dominated by nonthermal motion and that gas temperatures are well below those expected for O^{5 +} in collisional ionization equilibrium. This result reaffirms previous findings from studies of individual lines of sight but are at odds with expectations for a WHIM origin. At least half of the absorbers can be explained by a simple photoionization model. In addition, in some absorbers we find evidence for large variation in gas density/metallicity across components in individual absorbers. Comparisons of multiple associated metal species further show that under the assumption of the gas being photoionized by the metagalactic background radiation field, the absorbing clouds have gas densities <n_H > < - 2.9 and sizes L > 1 kpc . Finally, we compare our absorber selection with the results of other independent studies.

Based in part 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 NAS5-26555.