The Mean Metal-line Absorption Spectrum of Damped Lyα Systems in BOSS

We study the mean absorption spectrum of the Damped Ly alpha (DLA) population at z similar to 2.6 by stacking normalized, rest-frame-shifted spectra of similar to 27,000 DLA systems from the DR12 of the Baryon Oscillation Spectroscopic Survey (BOSS)/SDSS-III. We measure the equivalent widths of 50 individual metal absorption lines in five intervals of DLA hydrogen column density, five intervals of DLA redshift, and overall mean equivalent widths for an additional 13 absorption features from groups of strongly blended lines. The mean equivalent width of lowionization lines increases with NH I, whereas for high-ionization lines the increase is much weaker. The mean metal line equivalent widths decrease by a factor similar to 1.1-1.5 from z similar to 2.1 to z similar to 3.5, with small or no differences between low-and high-ionization species. We develop a theoretical model, inspired by the presence of multiple absorption components observed in high-resolution spectra, to infer mean metal column densities from the equivalent widths of partially saturated metal lines. We apply this model to 14 low-ionization species and to Al III, S III, Si III, C IV, Si IV, N V, and O VI. We use an approximate derivation for separating the equivalent width contributions of several lines to blended absorption features, and infer mean equivalent widths and column densities from lines of the additional species N I, Zn II, C II*, Fe III, and S IV. Several of these mean column densities of metal lines in DLAs are obtained for the first time; their values generally agree with measurements of individual DLAs from highresolution, high signal-to-noise ratio spectra when they are available.