Exploring the dust content of Sloan Digital Sky Survey Data Release 7 damped Lyman a systems at 2.15 ≤ zab < 5.2

We have studied a sample of 1084 intervening absorption systems with 2.15 =zab= 5.2, having in the spectra of quasi-stellar objects (QSOs) in Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7), with the aim of understanding the nature and abundance of the dust and the chemical abundances in the damped Lyman a (DLA) absorbers. Composite spectra were constructed for the full sample and several subsamples, chosen on the basis of absorber and QSO properties. Average extinction curves were obtained for the samples by comparing their geometric mean composite spectra with those of two samples of QSOs, matching in zem and i magnitude with the DLA sample, one sample without any absorbers along their lines of sight and the other without any DLAs along their lines of sight irrespective of the presence of other absorption systems. We also derived relative extinction curves of several pairs of subsamples. While the average reddening in the DLA sample is small, we find definite evidence for the presence of dust in subsamples based on absorber properties, in particular the strength of metal absorption lines. DLAs along lines of sight to QSOs which are not colour selected are found to be more dusty compared to those along the lines of sight to the more numerous colour-selected QSOs. From these studies and from the strengths of absorption lines in the composite spectra, we conclude that =10 per cent of the DLAs in SDSS DR7 cause significant reddening, have stronger absorption lines and have higher abundances as compared to the rest of the sample. The rest of the sample shows little reddening. While, due to the dominant colour selection method used to target QSOs in the SDSS DR7, this fraction of 10 per cent likely represents a lower limit for the global fraction of dusty DLAs at high z, it is also possible that the dust grain sizes at high redshifts are larger, giving rise to a flat extinction curve over the observed range of wavelengths.