Evidence for dust reddening in DLAs identified through CaII(H&K) absorption

We present a new sample of 31 Ca II (H&K) λλ3935, 3970 absorption line systems with 0.84 < zabs < 1.3 discovered in the spectra of Sloan Digital Sky Survey (SDSS) Data Release 3 quasars, together with an analysis of their dust content. The presence of Calcium absorption together with measurements of the Mg II λ2796, Fe II λ2600 and Mg I λ2853 lines lead to the conclusion that the majority of our systems are Damped Ly-α (DLA) absorbers. The composite spectrum in the rest frame of the absorber shows clear evidence for reddening. Large and Small Magellanic Cloud extinction curves provide satisfactory fits, with a best-fit E(B-V) of 0.06, while the Galactic dust extinction curve provides a poor fit due to the lack of a strong 2175Å feature. A trend of increasing dust content with equivalent width of Ca II is present. Monte Carlo techniques demonstrate that the detection of reddening is significant at > 99.99% confidence. The discovery of significant amounts of dust in a subsample of DLAs has direct implications for studies of the metallicity evolution of the universe and the nature of DLAs in relation to high redshift galaxies. The gas:dust ratio is discussed. Our results suggest that at least ∼40% of the Ca II absorption systems are excluded from the magnitude-limited SDSS quasar sample as a result of the associated extinction, a fraction similar to the upper limit deduced at higher redshifts from radio-selected surveys.