Upper Limits on Metals in Quasar Lyman-Alpha Forest Clouds: Absence of C IV Lines in Echelle Spectra

Recently Lu presented tentative evidence for C IV lines in QSO Lyman-alpha forest systems with strong lines. We have performed a similar search for C IV in our 10 km/sec echelle spectra of the bright QSO HS~1946+7658. We shifted the spectra to align the expected positions of the C IV lines in 65 Lyman-alpha systems, then added them. The resulting composite spectrum, equivalent to 390 hours of exposure time on the Lick 3-m telescope, has a signal-to-noise ratio of 80 per 0.025 Angstrom in the rest frame of the absorbers. We do not see any C IV lines down to a $2\sigma$ limit of W$(1548)\le 1.4$~m\AA, about one-fifth of the strength of the lines seen by Lu. The C IV lines which Lu saw must be restricted to rare Lyman-alpha systems with large H I column densities $\ge {10}^{14}$~cm${}^{-2}$, which are too rare to show C IV in our sample. More common Lyman-alpha systems with H I column densities of ${10}^{13}$ – ${10}^{14}$~cm${}^{-2}$ do not show C IV lines. If their ionization is H/H~I $= 10^4$thentheyhave[C\mathrm{/}H]$\leq -2.0$.