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We analyzed the spectroscopic and photometric database of the 5th datarelease of the Sloan Digital Sky Survey (SDSS) to search for evidence of thequasar reddening produced by dust embedded in intervening damped Ly alpha (DLA)systems. From a list of 5164 quasars in the interval of emission redshift 2.25</= z_e </= 3.5 and SDSS spectra with signal-to-noise ratio SNR >/= 4, we builtup an "absorption sample" of 248 QSOs with a single DLA system in the intervalof absorption redshift 2.2 < z_a </= 3.5 and a "pool" of 1959 control QSOswithout DLA systems or strong metal systems. For each QSO of the absorptionsample we extracted from the pool a subset of control QSOs that are closest inredshift and magnitude. The mean color of this subset was used as a zero pointto measure the "deviation from the mean color" of individual DLA-QSOs, Delta_i.The colors were measured using "BEST" ugriz SDSS imaging data. The mean colorexcess of the absorption sample, <E>, was estimated by averaging the individualcolor deviations Delta_i. We find <E(r-z)> = 27 +/- 9 x 10**(-3) mag and<E(g-z)> = 54 +/- 12 x 10**(-3) mag. These values are representative of thereddening of DLA systems at z_a ~ 2.7 in SDSS QSOs with limiting magnitude r=/~ 20.2. The detection of the mean reddening is confirmed by severalstatistical tests. Analysis of the results suggests an origin of the reddeningin dust embedded in the DLA systems, with an SMC-type extinction curve. Byconverting the reddening into rest-frame extinction, we derive a meandust-to-gas ratio <A_V/N(HI)> ~ 2 to 4 x 10**(-23) mag cm^2. This value is ~-1.25 dex lower than the mean dust-to-gas ratio of the Milky Way, in line withthe lower level of metallicity in the present DLA sample.

The color excess of quasars with intervening DLA systems- Analysis of the SDSS data release five