Constraining the Metallicity of the Low Density Lyman-Alpha Forest Using OVI Absorption

We present a systematic search for OVI(1032Å,1037Å) absorption in a Keck HIRES spectrum of the z = 3.62 quasar Q1422+231, with the goal of constraining the metallicity and ionization state of the low density intergalactic medium (IGM). Comparison of CIV absorption measurements to models of the Lyα forest based on cosmological simulations shows that absorbers with NHI ∼> 10 14.5cm−2 have a mean carbon abundance [C/H] ≈ −2.5, assuming a metagalactic photoionizing background with the spectral shape predicted by Haardt & Madau (1996, HM). In these models, lower column density absorption arises in lower density gas where most CIV is photoionized to CV. Therefore, OVI should be the most sensitive tracer of metallicity in Lyα absorbers with NHI ∼< 10 14.5cm−2. OVI lines lie at wavelengths heavily contaminated by Lyman series absorption, so we interpret the search results by comparing to carefully constructed, mock Q1422 spectra drawn from a hydrodynamic simulation of a Λ-dominated cold dark matter model. A search for deep, narrow absorption features yields only a few candidate OVI lines in the spectrum of Q1422. HI absorption blankets the position of the doublet companion line in each case, and the total number of narrow lines is statistically consistent with that in zero-metallicity artificial spectra. Artificial spectra generated with the HM background and [O/H] ∼> −2.5 predict too many narrow lines and are statistically inconsistent with the data. We also search for OVI associated with CIV systems, using the optical depth ratio technique of Songaila (1998). With this method we do find significant OVI absorption; matching the data requires [O/C] ≈ +0.5 and corresponding [O/H] ≈ −2.0. Taken together, the narrow line and optical depth ratio results imply that (a) the metallicity in the low density regions of the IGM is at least a