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

We analyzed the spectroscopic and photometric database of the 5th data release of the Sloan Digital Sky Survey (SDSS) to search for evidence of the quasar 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 built up an "absorption sample" of 248 QSOs with a single DLA system in the interval of absorption redshift 2.2 < z_a </= 3.5 and a "pool" of 1959 control QSOs without DLA systems or strong metal systems. For each QSO of the absorption sample we extracted from the pool a subset of control QSOs that are closest in redshift and magnitude. The mean color of this subset was used as a zero point to 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 color excess of the absorption sample, <E>, was estimated by averaging the individual color 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 the reddening 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 several statistical tests. Analysis of the results suggests an origin of the reddening in dust embedded in the DLA systems, with an SMC-type extinction curve. By converting the reddening into rest-frame extinction, we derive a mean dust-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 with the lower level of metallicity in the present DLA sample.