The warm-hot intergalactic medium at Z ∼ 2.2: Metal enrichment and ionization source

Abstract

Results are presented for our search for warm-hot gas towards the quasar Q 0329-385. We identify ten O VI systems of which two are within 5000 km s-1 of Zem and a third one should be of intrinsic origin. The seven remaining systems have H I column densities 1013.7 ≤ N(H I) ≤ 1015.6 cm-2. At least ∼1/3 of the individual O VI sub-systems have temperatures T < 1 × 105 K and cannot originate in collisionally ionized gas. Photoionization by a hard UV background field reproduces well the ionic ratios for metallicities in the range 10-2.5-10-0.5 solar, with possibly sub-solar N/C relative abundance. For [O/C] = 0, the sizes inferred for the O VI clouds are in some cases larger than the maximum extent implied by the Hubble flow. This constraint is fulfilled assuming a moderate overabundance of oxygen relative to carbon. For a soft UV ionizing spectrum, an overabundance of O/C is required, [O/C] ≈ 0.0-1.3. For a hard(soft) U spectrum and [O/C] = 0(1), the O VI regions have overdenskies ρ/ρ̄ ≈ 10-40.