Velocity - metallicity correlation for high - z DLA galaxies : Evidence for 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 ) > ∼ 20 and redshifts in the range 1.7 < z abs < 4.3. For each of the systems, we measured in an homogeneous manner the metallicities relative to Solar, [X/H] (with X = Zn, or S or Si), and the velocity widths of low-ionization line profiles, ∆V. We provide for the first time evidence for a correlation between DLA metallicity and line profile velocity width, which is detected at the 6.1σ significance level. This confirms the trend previously observed in a much smaller sample by Wolfe & Prochaska (1998). The best-fit linear relation is [X/H] = 1.55(±0.12) log ∆V − 4.33(±0.23) with ∆V expressed in km s −1. The slope of the DLA velocity-metallicity relation is the same within uncertainties between the higher (z abs > 2.43) and the lower (z abs ≤ 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 mean metallicity and mean 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.