A homogeneous sample of sub-damped Lyman α systems - IV. Global metallicity evolution

Abstract

An accurate method to measure the abundance of high-redshift galaxies involves the observation of absorbers along the line of sight towards a background quasar. Here, we present abundance measurements of 13 z ≥ 3 sub-damped Lyman α (sub-DLA) systems (quasar absorbers with H i column density in the range 19 < log N(H i) < 20.3 cm-2) based on high-resolution observations with the VLT UVES spectrograph. These observations more than double the amount of metallicity information for sub-DLAs available at z > 3. These new data, combined with other sub-DLA measurements from the literature, confirm the stronger evolution of metallicity with redshift for sub-DLAs than for the classical damped Lyman α absorbers. In addition, these observations are used to compute for the first time, using photoionization modelling in a sample of sub-DLAs, the fraction of gas that is ionized. Based on these results, we calculate that sub-DLAs contribute no more than 6 per cent of the expected amount of metals at z ∼ 2.5. We therefore conclude that, even if sub-DLAs are found to be more metal-rich than classical DLAs, their contribution is insufficient to solve the so-called 'missing-metals' problem. © 2007 The Authors.