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 Ly alpha (DLA) systems, may cause the background QSO to appear reddened. We investigate the extent of this potential reddening by comparing the optical-to-infrared colors of QSOs with and without intervening absorbers. Our QSO sample is based on the Complete Optical and Radio Absorption Line System ( CORALS) survey of Ellison and coworkers. The CORALS data set consists of 66 radio-selected QSOs at z(em) >= 2.2 with complete optical identifications. We have obtained near-simultaneous B- and K-band magnitudes for a subset of the CORALS sample and supplemented our observations with further measurements published in the literature. In total, we have B - K colors for 42 of the 66 QSOs, of which 14 have intervening DLA systems. To account for redshift-related color changes, the B - K colors are normalized using the Sloan Digital Sky Survey QSO composite. The mean normalized B - K color of the DLA subsample is +0.12, whereas the mean for the no-DLA sample is -0.10; both distributions have rms scatters of similar to 0.5. Neither a Student's t-test nor a Kolmogorov-Smirnov test indicate that there is any significant difference between the two color distributions. Based on simulations that redden the colors of QSOs with intervening DLA systems, we determine a reddening limit that corresponds to E(B - V) < 0.04 (SMC-like extinction) at 99% confidence (3 sigma), assuming that E( B - V) is the same for all DLA systems. Finally, we do not find any general correlation between absorber properties ( such as [Fe/Zn] or neutral hydrogen column density) and B - K color. The two reddest QSOs with DLA systems in our sample have H i column densities that differ from each other by an order of magnitude and moderate gas-to-dust ratios as inferred from chemical abundances. One of these two QSOs shows evidence for strong associated absorption from X-ray observations, an alternative explanation for its very red color. We conclude that the presence of intervening galaxies causes a minimal reddening of the background QSO.