Element abundances at high redshift: MIKE observations of sub-damped Lyman alpha absorbers at 1.7 \textless z \textless 2.4

International audienceWe present chemical abundance measurements from high-resolution observations of five sub-damped Lyman alpha absorbers (sub-DLAs) at 1.7 \textless z \textless 2.4 observed with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the 6.5-m Magellan II (Clay) telescope. Lines of Zn ii, Mg i, Mg ii, Al ii, Al iii, S ii, Si ii, Si iv, C ii, C ii*, C iv, Ni ii, Mn ii and Fe ii were detected and column densities were determined. The metallicity of the absorbing gas, inferred from the nearly undepleted element Zn, is in the range of \textless-0.95 to +0.25 dex for the five absorbers in our sample, with three of the systems being near-solar or supersolar. We also investigate the effect of ionization on the observed abundances using photoionization modelling. Combining our data with other sub-DLA and DLA data from the literature, we report the most complete existing determination of the metallicity versus redshift relation for sub-DLAs and DLAs. We confirm the suggestion from previous investigations that sub-DLAs are, on average, more metal rich than DLAs and may evolve faster. We also discuss relative abundances and abundance ratios in these absorbers. The more metal-rich systems show significant dust depletion levels, as suggested by the ratios [Zn/Cr] and [Zn/Fe]. For the majority of the systems in our sample, the [Mn/Fe] versus [Zn/H] trend is consistent with that seen previously for lower redshift sub-DLAs. We also measure the velocity width values for the sub-DLAs in our sample from unsaturated absorption lines of Fe ii lambda lambda 2344, 2374, 2600 A, and examine where these systems lie in a plot of metallicity versus velocity dispersion. Finally, we examine cooling rate versus H i column density in these sub-DLAs, and compare this with the data from DLAs and the Milky Way interstellar medium. We find that most of the systems in our sample show higher cooling rate values compared to those seen in the DLAs