The Mean Metal-line Absorption Spectrum of DLAs in BOSS
  We study the mean absorption spectrum of the Damped Lyman alpha population at
$z\sim 2.6$ by stacking normalized, rest-frame shifted spectra of $\sim
27\,000$ DLAs from the DR12 of BOSS/SDSS-III. We measure the equivalent widths
of 50 individual metal absorption lines in 5 intervals of DLA hydrogen column
density, 5 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 low-ionization lines increases with $N_{\rm HI}$,
whereas for high-ionization lines the increase is much weaker. The mean metal
line equivalent widths decrease by a factor $\sim 1.1-1.5$ from $z\sim2.1$ to
$z \sim 3.5$, 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 AlIII, SIII, SiIII,
CIV, SiIV, NV and OVI. 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 NI, ZnII, CII${}^{*}$, FeIII, and SIV. 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
high-resolution, high signal-to-noise ratio spectra when they are available.
