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

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

Aug 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...520..456e&link_type=abstract

The Astrophysical Journal, Volume 520, Issue 2, pp. 456-468.

Astronomy and Astrophysics

Astronomy

54

Galaxies: Formation, Galaxies: Intergalactic Medium, Galaxies: Quasars: Absorption Lines, Quasars: Individual (Apm 08279+5255)

Scientific paper

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 Lyalpha forest clouds. In agreement with previous work, we find that approximately 50% of Lyalpha clouds with hydrogen column densities of logN(H i)>=14.5 have associated weak C IV absorption, with logN(C iv)>~12.0, and we derive a median N(C iv)/N(H i)=1.4x10^-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 Lyalpha 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 Lyalpha lines with 13.5<=logN(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 lambda1548 lines with individual values of column density and velocity dispersion scaled appropriately from the values measured in the corresponding Lyalpha lines. We find that even if the typical value of N(C iv)/N(H >i)=1.4x10^-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 Lyalpha 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 Lyalpha 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 Lyalpha clouds with optical depths as low as tau(Lyalpha)=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 logN(H i)~=14.0. We conclude that the question of whether there is a uniform degree of metal enrichment in the Lyalpha 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.

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