Physical Conditions of Five O VI Absorption Systems Towards PG $1522+101$

Abstract

We present the analysis of five O VI absorbers identified across a redshift path of z $\sim (0.6-1.3)$ towards the background quasar PG $1522+101$ with information on five consecutive ionization stages of oxygen from O II to O VI. The combined $HST$ and $Keck$ spectra cover UV, redshifted EUV, and optical transitions from a multitude of ions spanning ionization energies in the range of $\sim (13-300)$ eV. Low ionization (C II, O II, Si II, Mg II) and very high ionization species (Ne VIII, Mg X) are non-detections in all the absorbers. Three of the absorbers have coverage of He I, in one of which it is a $>3\sigma$ detection. The kinematic structures of these absorbers are extracted from C IV detected in $HIRES$ spectra. The farthest absorber in our sample also contains the detections of Ne V and Ne VI. Assuming co-spatial absorbing components, the ionization models show the medium to be multiphased with small-scale density-temperature inhomogeneities that are sometimes kinematically unresolved. In two of the absorbers, there is an explicit indication of the presence of a warm gas phase ($T\gtrsim 10^5$ K) traced by O VI. In the remaining absorbers, the column densities of the ions are consistent with a non-uniform photoionized medium. The sub-solar [C/O] relative abundances inferred for the absorbers point at enrichment from massive Type II supernovae. Despite metal enrichment, the inferred wide range for [O/H] $\sim$ [$-2.1,+0.2$] amongst the absorbers along with their anti-correlation with the observed H I suggest poor small-scale mixing of metals with hydrogen in the regions surrounding galaxies and the IGM.

Accepted for publication in MNRAS. 19 Figures, 2 Tables. Additional figures and tables in the Appendix

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