The Observability of Metal Lines Associated with the Lyman-alpha Forest

We develop a prescription for characterizing the strengths of metal lines associated with Lyα forest absorbers (LYFAs) of a given neutral hydrogen column density NHI and metallicity [O/H]. This Line Observability Index (LOX) is line-specific and translates, for weak lines, into a measure of the equivalent width. It can be evaluated quickly for thousands of transitions within the framework of a given model of the Lyα forest, providing a ranking of the absorption lines in terms of their strengths and enabling model builders to select the lines that deserve more detailed consideration, i.e. those that should be detectable in observed spectra of a given resolution and signal-to-noise ratio. We compute the LOX for a large number of elements and transitions in two cosmological models of the Lyα forest at z ∼ 3 derived from hydrodynamic simulations of structure formation. We present results for a cold dark matter universe with a cosmological constant; an Ω = 1 cold dark matter model yields nearly identical results, and we argue more generally that the LOX predictions are insensitive to the specific choice of cosmology. We also discuss how the LOX depends on redshift and on model parameters such as the mean baryonic density and radiation field. We find that the OVI (1032 Å, 1038 Å) doublet is the best probe of the metallicity in low column density LYFAs (NHI ≈ 10 cm). Metallicities down to [O/H] ∼ -3 yield OVI absorption features that should be detectable in current high-quality spectra, provided that the expected position of the OVI feature is not contaminated by HI absorption. The strongest transitions in lower ionization states of oxygen are OV(630 Å), OIV(788 Å), and OIII(833 Å). These absorption lines are all predicted to be stronger than the OVI feature, but even at redshifts 3− 4 they will have to be observed in the ultraviolet, and they are extremely difficult to detect with present UV instruments, such as the Space Telescope Imaging Spectrograph (STIS). At lower Presidential Faculty Fellow