Strong z ∼ 0.5 OVI absorption towards PKS 0405-123: implications for ionization and metallicity of the Cosmic Web

We present observations of the intervening OVI absorption-line system at z(abs) = 0.495096 towards the quasi-stellar object (QSO) PKS 0405-123 (z(em) = 0.5726) obtained with the Far Ultraviolet Spectroscopic Explorer and Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. In addition to strong O VI, with logN(OVI) = 14.47 +/- 0.02, and moderate HI, with log N(HI) = 14.29 +/- 0.10, this absorber shows absorption from CIII, NIV, OIV and OV, with upper limits for another seven ions. The large number of available ions allows us to test ionization models usually adopted with far fewer constraints. We find that the observed ionic column densities cannot be matched by single-temperature collisional ionization models, in or out of equilibrium. Photoionization models can match all of the observed column densities, including OVI. If one assumes photoionization by an ultraviolet (UV) background dominated by QSOs, the metallicity of the gas is [O/H] approximate to -0.15, while if one assumes a model for the UV background with contributions from ionizing photons escaping from galaxies the metallicity is [O/H] approximate to -0.62. Both give [N/O] approximate to -0.6 and [C/H] similar to -0.2 to similar to-0.1, though a solar C/O ratio is not ruled out. The choice of ionizing spectrum is poorly constrained and leads to systematic abundance uncertainties of approximate to 0.5 dex, despite the wide range of available ions. Multiphase models with a contribution from both photoionized gas (at T similar to 10(4) K) and collisionally ionized gas [at T similar to (1-3) x 10(5) K] can also match the observations for either assumed UV background giving very similar metallicities. We do not detect Ne VIII or Mg x absorption. The limit on Ne VIII/OVI < 0.21 (3 sigma) is the lowest yet observed. Thus, this absorber shows no firm evidence of the 'warm-hot intergalactic medium' at T similar to (0.5-3) x 10(6) K thought to contain a significant fraction of the baryons at low redshift. The OVI in this system is not necessarily a reliable tracer of the warm-hot intergalactic medium given the ambiguity in its origins. We present limits on the total column of warm-hot gas in this absorber as a function of temperature. This system would be unlikely to provide detectable X-ray absorption in the ions OVII or OVIII even if it resided in front of the brighter X-ray sources in the sky.