A homogeneous sample of sub-damped Lyman alpha systems - II. Statistical, kinematic and chemical properties

Abstract

Damped Lyman α systems (DLAs), with N(H i) > 2 × 1020 atom cm−2, observed in the spectra of quasars have allowed us to quantify the chemical content of the Universe over cosmological scales. Such studies can be extended to lower column densities, in the sub-DLA range [1019 < N(H i) < 2 × 1020 atom cm−2], which are systems believed to contain a large fraction of the neutral hydrogen at z > 3.5. In this paper, we use a homogeneous sample of sub-DLAs from the European Southern Oberservatory (ESO) Ultraviolet–Visual Echelle Spectrograph (UVES) archives presented in Paper I, to determine observationally for the first time the shape of the column density distribution, ƒ(N), down to N(H i) = 1019 atom cm−2. The results are in good agreement with the predictions from Péroux et al. We also present the kinematic and clustering properties of this survey of sub-DLAs, which appear to be marginally different from the DLAs. We compare low- and high-ionization transition widths and find that the properties of the sub-DLAs span roughly the parameter space of DLAs. We also find hints of an increase of metallicity in systems with larger velocity widths in the metal lines, although the statistical significance of this result is low. Then we analyse the chemical content of this sample in conjunction with a compilation of abundances from 72 DLAs taken from the literature. As previously reported, the individual metallicities traced by [Fe/H] of these systems evolve mildly with redshift. Moreover, we analyse the H i column-density-weighted mean abundance, which is believed to be an indicator of the metallicity of the Universe. Although the number statistics is limited in the current sample, the results suggest a slightly stronger evolution of this quantity in the sub-DLA range. The effect is predominant at z < 2 and most of the evolution observed lies in this redshift range. Observational arguments support the hypothesis that the evolution we probe in the sub-DLA range is not due to their lower dust content. Therefore, these systems might be associated with a different class of objects, which better trace the overall chemical evolution of the Universe. Finally, we present abundance ratios of [Si/Fe], [O/Fe], [C/Fe] and [Al/Fe] for sub-DLAs in conjunction with DLA measurements from the literature. The elemental ratios in sub-DLAs are comparable with those from DLAs. It is difficult to decipher whether the observed values are the effect of nucleosynthesis or are due to differential dust depletion. The metallicities are compared with two different sets of models of galaxy evolution in order to provide constraints on the morphology of quasar absorbers.