Astronomy & Astrophysics

A&A 457, 71-78 (2006)
DOI: 10.1051/0004-6361:20054242

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

C. Ledoux¹, P. Petitjean^{2, 3}, J. P. U. Fynbo⁴, P. Møller⁵ and R. Srianand⁶

¹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago 19, Chile
    e-mail: cledoux@eso.org
² Institut d'Astrophysique de Paris - UMR 7095 CNRS & Université Pierre et Marie Curie, 98bis Boulevard Arago, 75014 Paris, France
    e-mail: petitjean@iap.fr
³ LERMA, Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, France
⁴ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
    e-mail: jfynbo@astro.ku.dk
⁵ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
    e-mail: pmoller@eso.org
⁶ Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganesh Khind, Pune 411 007, India
    e-mail: anand@iucaa.ernet.in

(Received 23 September 2005 / Accepted 22 May 2006 )

Abstract
We used our database of VLT-UVES quasar spectra to build up a sample of 70 Damped Lyman- $\alpha$ (DLA) or strong sub-DLA systems with total neutral hydrogen column densities of $\log N($ H I $)\ga 20$ and redshifts in the range $1.7<z_{\rm abs}<4.3$ . For each of the systems, we measured the metallicities relative to solar in an homogeneous manner, [X/H] (with ${\rm X}={\rm Zn}$ , or S or Si), and the velocity widths of low-ionization line profiles, $\Delta V$ . For the first time, we provide evidence for a correlation between DLA metallicity and line profile velocity width, which is detected at the $6.1\sigma$ significance level. This confirms the trend previously observed in a much smaller sample by Wolfe & Prochaska (1998). The best-fit linear relation is $[{\rm X}/{\rm H}]=1.55(\pm 0.12)\log\Delta V -4.33(\pm 0.23)$ , with $\Delta V$ expressed in km s^-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.

Key words: galaxies: halos -- galaxies: high-redshift -- galaxies: ISM -- quasars: absorption lines -- cosmology: observations