The Optical-Infrared Colors of CORALS QSOs: Searching for Dust Reddening Associated with High-Redshift Damped Lyα Systems

The presence of dust in quasar absorbers, such as damped Lyman alpha (DLA)systems, may cause the background QSO to appear reddened. We investigate theextent of this potential reddening by comparing the optical-to-infrared (IR)colors of QSOs with and without intervening absorbers. Our QSO sample is basedon the Complete Optical and Radio Absorption Line System (CORALS) survey ofEllison et al (2001). We have obtained near-simultaneous B and K bandmagnitudes for subset of the CORALS sample and supplemented our observationswith further measurements published in the literature. To account forredshift-related color changes, the B-K colors are normalized using the SloanDigital Sky Survey (SDSS) QSO composite. The mean normalized B-K color of theDLA sub-sample is +0.12, whereas the mean for the no-DLA sample is -0.10; bothdistributions have RMS scatters ~0.5. Neither a student's T-test nor a KS testindicate that there is any significant difference between the two colordistributions. Based on simulations which redden the colors of QSOs withintervening DLAs, we determine a reddening limit which corresponds to E(B-V) <0.04 (SMC-like extinction) at 99% confidence (3 sigma), assuming that E(B-V) isthe same for all DLAs. Finally, we do not find any general correlation betweenabsorber properties (such as [Fe/Zn] or neutral hydrogen column density) andB-K color. One of these two QSOs shows evidence for strong associatedabsorption from X-ray observations, an alternative explanation for its very redcolor. We conclude that the presence of intervening galaxies causes a minimalreddening of the background QSO.Comment: Accepted for publication in A