A metal-line strength indicator for Damped Lyman Alpha (DLA) systems at
  low signal-to-noise
  The Baryon Oscillation Spectroscopic Survey of SDSS-III has provided an
unprecedentedly large sample of Damped \lya systems (DLAs), the largest
repositories of neutral hydrogen in the Universe. This DLA sample has been used
to determine the DLA bias factor from their cross-correlation with the \lya
forest absorption in \cite{FontRibera2012,Perez2018}, showing that DLAs are
associated with relatively massive halos. However, the low resolution and
signal-to-noise of BOSS spectra do not allow precise measurements of the DLA
metal lines. We define a metal strength parameter, $S$, based on combining
equivalent widths of 17 metal lines, that can be measured with an optimal
signal-to-noise ratio for individual DLAs in BOSS spectra, allowing for the
classification of the DLA population into subgroups of different $S$. We
present the distribution of this DLA metal strength and the dependence of its
mean value on $N_{\rm HI}$ and redshift. We search for systematic effects and
variations in the catalogue purity by examining the dependence of the $S$
distribution on the spectral signal-to-noise and the estimated error on $S$. A
catalogue of DLAs with measured equivalent widths for the selected 17 metal
lines and the value of $S$ is made publicly available, which will be used to
measure the dependence of the DLA bias factor on the $S$ parameter. The
relation of the metal strength on the gas metal abundances and velocity
dispersion can be constrained by studying the stacked metal absorption spectra
of DLAs as a function of $S$, allowing for future determinations of the
dependence of the bias factor on the metallicity and velocity dispersion of
DLAs.
