Strong z ∼ 0.5 O vi absorption towards PKS 0405−123: implications for ionization and metallicity of the Cosmic Web★

Abstract

We present observations of the intervening O VI absorption-line system at zabs = 0.495 096 towards the quasi-stellar object (QSO) PKS 0405−123 (zem = 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 log N(O VI) = 14.47 ± 0.02, and moderate H I, with log N (H I) = 14.29 ± 0.10, this absorber shows absorption from C III, N IV ,O IV and O V, 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 O VI. If one assumes photoionization by an ultraviolet (UV) background dominated by QSOs, the metallicity of the gas is [O/H] ≈− 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] ≈− 0.62. Both give [N/O] ∼− 0.6 and [C/H] ∼− 0.2 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 ≈0.5 dex, despite the wide range of available ions. Multiphase models with a contribution from both photoionized gas (at T ∼ 10 4 K) and collisionally ionized gas [at T ∼ (1−3) × 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/O VI < 0.21 (3σ )i s the lowest yet observed. Thus, this absorber shows no firm evidence of the ‘warm-hot intergalactic medium’ at T ∼ (0.5–3) × 10 6 K thought to contain a significant fraction of the baryons at low redshift. The O VI 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 O VII or O VIII even if it resided in front of the brighter X-ray sources in the sky.