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We develop a prescription for characterizing the strengths of metal lines associated with Lyα forest absorbers (LYFAs) of a given neutral hydrogen column density NH I 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 O VI (λλ1032, 1038) doublet is the best probe of the metallicity in low column density LYFAs (NH I 1014.5 cm-2). Metallicities down to [O/H] ~ -3 yield O VI absorption features that should be detectable in current high-quality spectra, provided that the expected position of the O VI feature is not contaminated by H I absorption. The strongest transitions in lower ionization states of oxygen are O V (λ630), O IV (λ788), and O III (λ833). These absorption lines are all predicted to be stronger than the O VI 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 redshifts, detection of these lines may be possible in STIS spectra of the very brightest QSOs, while one may have to wait for next-generation instruments such as the Cosmic Origins Spectrograph (COS) to detect such lines in a number of high-redshift QSOs.

The strongest metal lines with rest frame wavelength larger than 912 Å associated with higher column density LYFAs at z ~ 3 are C III (λ977) and Si III (λ1206.5), which peak at NH I ~ 1017 cm-2. Of the lines with rest wavelengths λr > 1216 Å, which can potentially be observed redward of the Lyα forest, the C IV (λλ1548, 1551) doublet is expected to dominate in all LYFAs, regardless of the value of NH I. We argue that C IV and C II absorption may peak in different spatial regions, and that comparison of single-phase models of the C IV/C II ionization ratios with observed C IV/C II column density ratios can lead to an overestimate of the ionization parameter in the central parts of the absorbers.


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