The dust content of damped Lyman-alpha systems in the Sloan Digital Sky
  Survey
  The dust-content of damped Lyman-alpha systems (DLAs) is an important
observable for understanding their origin and the neutral gas reservoirs of
galaxies. While the average colour-excess of DLAs, E(B-V), is known to be <15
milli-magnitudes (mmag), both detections and non-detections with ~2 mmag
precision have been reported. Here we find 3.2-sigma statistical evidence for
DLA dust-reddening of 774 Sloan Digital Sky Survey (SDSS) quasars by comparing
their fitted spectral slopes to those of ~7000 control quasars. The
corresponding E(B-V) is 3.0 +/- 1.0 mmag, assuming a Small Magellanic Cloud
(SMC) dust extinction law, and it correlates strongly (3.5-sigma) with the
metal content, characterised by the SiII1526 absorption-line equivalent width,
providing additional confidence that the detection is due to dust in the DLAs.
Evolution of E(B-V) over the redshift range 2.1 < z < 4.0 is limited to <2.5
mmag per unit redshift (1-sigma), consistent with the known, mild DLA
metallicity evolution. There is also no apparent relationship with neutral
hydrogen column density, N(HI), though the data are consistent with a mean
E(B-V)/N(HI) = (3.5 +/- 1.0) x 10^{-24} mag cm^2, approximately the ratio
expected from the SMC scaled to the lower metallicities typical of DLAs. We
implement the SDSS selection algorithm in a portable code to assess the
potential for systematic, redshift-dependent biases stemming from its magnitude
and colour-selection criteria. The effect on the mean E(B-V) is negligible (<5
per cent) over the entire redshift range of interest. Given the broad potential
usefulness of this implementation, we make it publicly available.
