Exploring the dust content of SDSS DR7 damped Lyman alpha systems at
  2.15$\le z_{ab}< $5.2
  We have studied a sample of 1084 intervening absorption systems with 2.15$\le
z_{ab}\le$5.2, having log(N$_{\rm HI}$) $>$ 20.0 in the spectra of 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 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
z$_{em}$ 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. 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 $\le$ 10% 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. Due to the
dominant color selection method used to target QSOs in the SDSS DR7, this
fraction of 10% likely represents a lower limit for the global fraction of
dusty DLAs at high-z.
