Velocity-metallicity correlation for high-z DLA galaxies: evidence of a mass-metallicity relation?

We used our database of VLT-UVES quasar spectra to build up a sample of 70 Damped Lyman- α (DLA) or strong sub-DLA systems with total neutral hydrogen column densities of $\log N($H i$)\ga 20$ and redshifts in the range $1.7 -1 . The slope of the DLA velocity-metallicity relation is the same within uncertainties between the higher ($z_{\rm abs}>2.43$) and the lower ($z_{\rm abs}\le 2.43$) redshift halves of our sample. However, the two populations of systems are statistically different. There is a strong redshift evolution in the sense that the median metallicity and median velocity width increase with decreasing redshift. We argue that the existence of a DLA velocity-metallicity correlation, over more than a factor of 100 spread in metallicity, is probably the consequence of an underlying mass-metallicity relation for the galaxies responsible for DLA absorption lines. Assuming a simple linear scaling of the galaxy luminosity with the mass of the dark-matter halo, we find that the slope of the DLA velocity-metallicity relation is consistent with that of the luminosity-metallicity relation derived for local galaxies. If the galaxy dynamical mass is indeed the dominant factor setting up the observed DLA velocity-metallicity correlation, then the DLA systems exhibiting the lowest metallicities among the DLA population should, on average, be associated with galaxies of lower masses (e.g., gas-rich dwarf galaxies). In turn, these galaxies should have the lowest luminosities among the DLA galaxy population. This could explain the difficulties of detecting high-redshift DLA galaxies in emission.