Understanding the strong intervening OVI absorber at z$_{abs}$ ~0.93 towards PG1206+459

Abstract

We have obtained new observations of the partial Lyman limit absorber at \zabs$=0.93$ towards quasar PG~1206+459, and revisit its chemical and physical conditions. The absorber, with $ N(HI) \sim 10^{17.0}$ ~\sqcm\ and absorption lines spread over $\gtrsim$1000~\kms\ in velocity, is one of the strongest known OVI absorbers at $\log N(OVI)=$15.54$\pm$0.17. Our analysis makes use of the previously known low-(e.g. \MgII), intermediate-(e.g. SiIV), and high-ionization (e.g., CIV, NV, NeVIII) metal lines along with new $HST/$COS observations that cover OVI, and an $HST/$ACS image of the quasar field. Consistent with previous studies, we find that the absorber has a multiphase structure. The low-ionization phase arises from gas with a density of $\log (n_{\rm H}/\rm cm^{-3})\sim-2.5$ and a solar to super-solar metallicity. The high-ionization phase stems from gas with a significantly lower density, i.e. $\log (n_{\rm H}/\rm cm^{-3}) \sim-3.8$, and a near-solar to solar metallicity. The high-ionization phase accounts for all of the absorption seen in CIV, NV, and OVI. We find the the detected \NeVIII, reported by \cite{Tripp2011}, is best explained as originating in a stand-alone collisionally ionized phase at $T\sim10^{5.85}~\rm K$, except in one component in which both OVI and NeVIII can be produced via photoionization. We demonstrate that such strong OVI absorption can easily arise from photoionization at $z\gtrsim1$, but that, due to the decreasing extragalactic UV background radiation, only collisional ionization can produce large OVI features at $z\sim0$. The azimuthal angle of $\sim88$\degree\ of the disk of the nearest ($\rm 68~kpc$) luminous ($1.3L_*$) galaxy at $z_{\rm gal}=0.9289$, which shows signatures of recent merger, suggests that the bulk of the absorption arises from metal enriched outflows.