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 α system (DLA) survey. The sample contains 99 DLAs towards 89 quasars selected from the SDSS DR5 DLA sample in a uniform way. We analyse the metallicities and kinematic diagnostics, including the velocity width of 90 per cent of the optical depth, Δv₉₀, and the equivalent widths of the Si II λ1526 (W_λ1526), C IV λ1548 and Mg II λ2796 transitions. To avoid strong line-saturation effects on the metallicities measured in medium-resolution spectra (FWHM ∼ 71 km s^-1), we derived metallicities from metal transitions which absorbed at most 35 per cent of the quasar continuum flux. We find the evolution in cosmic mean metallicity of the sample, <Z> = (-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 Δv₉₀ and W_λ1526 values. While this result may seem to conflict with other large surveys that have detected significant metallicity evolution, such as Rafelski et al. who found <Z> = (-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. estimate.