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 a (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 greater than or similar to 4, we built up an absorption sampleof 248 QSOs with a single DLA system in the interval of absorption redshift 2.2 < z(a) <= 3.5 and a poolof 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 za approximate to 2.7 in SDSS QSOs with limiting magnitude r similar or equal to 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 < AV/N(H-I)> approximate to 2 to 4 x 10(-23) mag cm(2). This value is similar or equal to-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.