A STIS Survey for OVI Absorption Systems at 0.12 < z < 0.5 II.: Physical Conditions of the Ionised Gas

TL;DR

This study catalogs 27 low-redshift O VI absorbers, analyzing their physical conditions, and finds that non-thermal motions dominate line widths, with many absorbers explained by photoionization rather than collisional ionization.

Contribution

First comprehensive survey of low-redshift O VI absorbers using HST/STIS data, analyzing their physical conditions and ionization mechanisms.

Findings

Line widths dominated by non-thermal motions.

Gas temperatures below collisional ionization equilibrium expectations.

At least half of absorbers explained by photoionization models.

Abstract

We present a complete catalogue of 27 O VI absorbers at low redshift (0.12 < z < 0.5) from a blind survey of 16 QSO echelle spectra in the HST/STIS data archive. These absorbers are identified based only upon matching line profiles and the expected doublet ratio between the \lambda\lambda 1031, 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 which are well-aligned in velocity-space, we derive gas temperatures and non-thermal broadening values, b_{nt}. We show that in all 16 cases considered the observed line width is dominated by non-thermal 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 metagalacitic 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.