A metal-line strength indicator for Damped Lyman Alpha (DLA) systems at low signal-to-noise

摘要

The Baryon Oscillation Spectroscopic Survey of SDSS-III has provided anunprecedentedly large sample of Damped \lya systems (DLAs), the largestrepositories of neutral hydrogen in the Universe. This DLA sample has been usedto determine the DLA bias factor from their cross-correlation with the \lyaforest absorption in \cite{FontRibera2012,Perez2018}, showing that DLAs areassociated with relatively massive halos. However, the low resolution andsignal-to-noise of BOSS spectra do not allow precise measurements of the DLAmetal lines. We define a metal strength parameter, $S$, based on combiningequivalent widths of 17 metal lines, that can be measured with an optimalsignal-to-noise ratio for individual DLAs in BOSS spectra, allowing for theclassification of the DLA population into subgroups of different $S$. Wepresent the distribution of this DLA metal strength and the dependence of itsmean value on $N_{\rm HI}$ and redshift. We search for systematic effects andvariations in the catalogue purity by examining the dependence of the $S$distribution on the spectral signal-to-noise and the estimated error on $S$. Acatalogue of DLAs with measured equivalent widths for the selected 17 metallines and the value of $S$ is made publicly available, which will be used tomeasure the dependence of the DLA bias factor on the $S$ parameter. Therelation of the metal strength on the gas metal abundances and velocitydispersion can be constrained by studying the stacked metal absorption spectraof DLAs as a function of $S$, allowing for future determinations of thedependence of the bias factor on the metallicity and velocity dispersion ofDLAs.