The early build-up of dust in galaxies: A study of Damped Ly alpha Systems

We present a study of the early build-up of dust in high redshift galaxies. The study is based on the analysis of 38 Damped Ly alpha systems (DLAs) for which we derive the fraction of iron atoms in dust form, f_{Fe}. The sample is representative of metal-poor galaxies in the redshift range 0.6 </= z </= 3.4 selected on the basis of their absorption HI column density (N(HI) >/= 2 x 10^{20} atoms cm^{-2}). We find that the dust fraction increases with metallicity, from f_{Fe}~0 at [Fe/H] ~ -2 dex, up to f_{Fe} ~ 0.9 at solar metallicity; the increase is fast below [Fe/H] ~ -1 dex and mild at higher metallicities. We also find some evidence for an increase of f_{Fe} with cosmic time; a large fraction of the systems younger than ~3 Gyr has f_{Fe} </~ 0.5. These results indicate the dust-to-metal ratio increases in the course of chemical evolution, at variance with the hypothesis of an approximately constant dust-to-metal ratio, commonly adopted in models of galactic evolution. This hypothesis is consistent with local and high-redshift data only when the metallicity is relatively high ([Fe/H] >/~ -1 dex). The results of this work suggest that the main mechanisms of dust formation may be rather sensitive to the level of metallicity attained by a galaxy in the course of its chemical evolution. A metallicity-dependent dust production by SNe II seems to be the most promising mechanism for explaining the rise of f_{Fe} at [Fe/H] </~ -1 dex.Comment: Accepted for publication on Astronomy & Astrophysics; 13 pages, 5 figure

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 /= 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, , was estimated by averaging the individual color deviations Delta_i. We find = 27 +/- 9 x 10**(-3) mag and = 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 ~ 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.Comment: Accepted for publication on Astronomy & Astrophysics, 17 pages, 10 figure