The Observability of Metal Lines Associated with the Lyman-alpha Forest
Abstract
We develop a prescription for characterizing the strengths of metal lines associated with Lyman-alpha forest absorbers (LYFAs) of a given neutral hydrogen column density N_HI and metallicity [Fe/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 hundreds of lines within the framework of a given model of the Lyman-alpha forest, providing a ranking of the 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 a cosmological model of the Lyman-alpha forest derived from a hydrodynamic simulation of structure formation in a cold dark matter universe at z=3, and we 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 (1032,1038) doublet is the best probe of the metallicity in low column density LYFAs N_{HI} \approx 10^{14.5} cm^{-2}). Metallicities down to [O/H] \sim -3 ([Fe/H] \sim -3.5 with the assumed [O/Fe] ratio) 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 metal lines associated with higher column density LYFAs at z \sim 3 are CIII (977) and SiIII (1206.5), which peak at N_HI \sim 10^{17} cm^{-2}. Of the lines with rest wavelengths \lambda_r > 1216, which can potentially be observed redwards of the \lya forest, the CIV(1548,1551) doublet is expected to dominate in all LYFAs, regardless of the value of N_HI. (Abridged)Keywords
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