HIRES Spectroscopy of APM 08279+5255: Metal Abundances in the Lyα Forest

We present high-S/N echelle spectra of the recently discovered ultraluminous QSO APM 08279+5255 and use these data to reexamine the abundance of carbon in Ly? forest clouds. In agreement with previous work, we find that approximately 50% of Ly? clouds with hydrogen column densities of log N (H I) ? 14.5 have associated weak C IV absorption, with log N(C IV) 12.0, and we derive a median N(C IV)/N(H I) = 1.4 ? 10-3. The agreement with earlier estimates of this ratio may be somewhat fortuitous, however, because we show that previous analyses have probably overestimated the number of Ly? clouds that should be included in this statistic. We then investigate whether there is any C IV absorption associated with lower H I column densities by stacking 51 C IV regions corresponding to 51 Ly? lines with 13.5 ? log N(H I) ? 14.0. The coadded spectrum has a S/N 580 but shows no composite C IV absorption. In order to understand the significance of this nondetection, we have stacked together 51 theoretical C IV ?1548 lines with individual values of column density and velocity dispersion scaled appropriately from the values measured in the corresponding Ly? lines. We find that even if the typical value of N(C IV)/N(H I) = 1.4 ? 10-3 applies to these lower column density clouds, the corresponding signal in the stacked C IV region is smeared by the likely random difference in redshift between C IV and Ly? absorption and becomes very difficult to recognize. This seems to be a fundamental limitation of the stacking method, which may well explain why in the past it has led to underestimates of the metallicity of the Ly? forest. We also analyze our spectra with the pixel-by-pixel optical depth technique recently developed by Cowie & Songaila and find evidence for net C IV absorption in Ly? clouds with optical depths as low as ?(Ly?) = 0.5-2, as these authors did. However, we show with simulations that even this method requires higher sensitivities than reached up to now to be confident that the ratio N(C IV)/N(H I) remains constant down to column densities below log N(H I) 14.0. We conclude that the question of whether there is a uniform degree of metal enrichment in the Ly? forest at all column densities has yet to be fully answered. Future progress in this area will probably require concerted efforts to push further the detection limit for C IV lines in selected bright QSOs.