Understanding the strong intervening OVI absorber at z_abs ~ 0.93 towards PG1206+459

We have obtained new observations of the partial Lyman limit absorber at z_abs=0.93 towards quasar PG 1206+459, and revisit its chemical and physical conditions. The absorber, with N(H I) ~ 10^17.0 cm^-2 and absorption lines spread over ≳1000 km s^-1 in velocity, is one of the strongest known OVI absorbers at logN(O VI) = 15.54±0.17. Our analysis makes use of the previously known low- (e.g. MgII), intermediate- (e.g. Si IV), and high-ionization (e.g. CIV, NV, Ne VIII) metal lines along with new Hubble Space Telescope (HST)/Cosmic Origins Spectrograph (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(nH/cm^-3) ~ -2.5 and a solar to supersolar metallicity. The high-ionization phase stems from gas with a significantly lower density, i.e. log(nH/cm^-3) ~ -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 Ne VIII, reported by Tripp et al. (2011), is best explained as originating in a stand-alone collisionally ionized phase at T ~ 105.85 K, except in one component in which both OVI and Ne VIII can be produced via photoionization. We demonstrate that such strong OVI absorption can easily arise from photoionization at z ≳ 1, but that, due to the decreasing extragalactic UV background radiation, only collisional ionization can produce large OVI features at z ~ 0. The azimuthal angle of ~88° of the disc of the nearest (68 kpc) luminous (1.3L*) galaxy at z_gal =0.9289, which shows signatures of recent merger, suggests that the bulk of the absorption arises from metal enriched outflows.