H i content, metallicities and spin temperatures of damped and sub-damped Lyα systems in the redshift desert (0.6 < z_abs < 1.7)*

Abstract

The H i 21-cm absorption optical depth and the N(H i) column density derived from Lyα absorption can be combined to yield the spin temperature (Ts) of intervening damped Lyα systems (DLAs). Although spin temperatures have been determined for samples of DLAs with zabs < 0.6 and zabs > 1.7, the intermediate regime currently contains only two Ts measurements, leading to a �redshift desert� that spans 4 Gyr of cosmic time. To connect the low- and high-redshift regimes, we present observations of the Lyα line of six 0.6 < zabs < 1.7 H i 21-cm absorbers. The data set is complemented by both low-frequency Very Long Baseline Array observations (to derive the absorber covering factor, f) and optical echelle spectra from which metal abundances are determined. Our data set therefore not only offers the largest statistical study of H i 21-cm absorbers to date, and bridges the redshift desert, but is also the first to use a fully f-corrected data set to look for metallicity-based trends.The metallicities of H i 21-cm absorbers are consistent with those of optically selected DLAs at the same redshift. In agreement with trends found in Galactic sightlines, we find that the lowest column density absorbers tend to be dominated by warm gas. In the DLA regime, spin temperatures show a wider range of values than Galactic data, as may be expected in a heterogeneous galactic population. However, we find that low-metallicity DLAs are dominated by small cold gas fractions and only absorbers with relatively high metallicities exhibit significant fractions of cold gas. Using a compilation of H i 21-cm absorbers which are selected to have covering-factor-corrected spin temperatures, we confirm an anticorrelation between metallicity and Ts at 3.4� significance. Finally, one of the DLAs in our sample is a newly discovered H i 21-cm absorber (at zabs= 0.602 towards J1431+3952), which we find to have the lowest f-corrected spin temperature yet reported in the literature: K. The observed distribution of Ts and metallicities in DLAs and the implications for understanding the characteristics of the interstellar medium in high-redshift galaxies are discussed.