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

Abstract

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 H i and redshifts in the range . For each of the systems, we measured the metallicities relative to solar in an homogeneous manner, [X/H] (with , or S or Si), and the velocity widths of low-ionization line profiles, . For the first time, we provide evidence for a correlation between DLA metallicity and line profile velocity width, which is detected at the significance level. This confirms the trend previously observed in a much smaller sample by Wolfe & Prochaska (1998). The best-fit linear relation is , with expressed in km s-1. The slope of the DLA velocity-metallicity relation is the same within uncertainties between the higher () and the lower () 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.