The Magellan uniform survey of damped Lyman α systems – I. Cosmic metallicity evolution

We present the chemical abundance measurements of the first large, medium-resolution, uniformly selected damped Lyman- system (DLA) survey. The sample contains 99 DLAs towards 89 quasars selected from the SDSS DR5 DLA sample in a uniform way. We analyze the metallicities and kinematic diagnostics, including the velocity width of 90% of the optical depth, v90, and the equivalent widths of the Siii 1526 (W 1526), Civ 1548 and Mgii 2796 transitions. To avoid strong line-saturation e ects on the metallicities measured in medium-resolution spectra (FWHM 71 km s 1 ), we derived metallicities from metal transitions which absorbed at most 35 % of the quasar continuum flux. We find the evolution in cosmic mean metallicity of the sample,hZi = ( 0.04 0.13)z (1.06 0.36), consistent with no evolution over the redshift range z [2:2; 4:4], but note that the majority of our sample falls at z [2:2; 3:5]. The apparent lack of metallicity evolution with redshift is also seen in a lack of evolution in the median v90 and W 1526 values. While this result may seem to conflict with other large surveys that have detected significant metallicity evolution, such as Rafelski et al. (2012) who foundhZi = ( 0.22 0.03)z (0.65 0.09) over z [0; 5], several tests show that these surveys are not inconsistent with our new result. However, over the smaller redshift range covered by our uniformly-selected sample, the true evolution of the cosmic mean metallicity in DLAs may be somewhat flatter than the Rafelski et al. (2012) estimate.