Evidence for dust reddening in DLAs identified through CaII H&K   absorption
  We present a new sample of 31 CaII(H&K) 3935,3970 absorption line systems with 0.84<z_abs<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 MgII 2796, FeII 2600 and MgI 2853 lines lead to the conclusion that the majority of our systems are Damped Ly-alpha (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 2175A feature. A trend of increasing dust content with equivalent width of CaII 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 CaII 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. 
Vivienne Wild and Paul C. Hewett (Institute of Astronomy, Cambridge)