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 Lyalpha forest systems with strong lines. We have performed a similar search for C IV in our 10 km s-1 resolution 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 Lyalpha systems and then added them. The resulting composite spectrum, equivalent to 390 hr 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) less-than-or-equal-to 1.4 mangstrom, about one fifth of the strength of the lines seen by Lu. The C iv lines that Lu saw must be restricted to rare Lyalpha systems with large H I column densities greater-than-or-equal-to 10(14) cm-2, which are too rare to show C IV in our sample. More common Lyalpha 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) then they have [C/H] less-than-or-equal-to -2.0.