Dust reddening in Sloan Digital Sky Survey quasars

Abstract

We explore the form of extragalactic reddening toward quasars using a sample of 9566 quasars with redshifts 0 < z < 2.2 and accurate optical colors from the Sloan Digital Sky Survey (SDSS). We confirm that dust reddening is the primary explanation for the red "tail'' of the color distribution of SDSS quasars. Our fitting to five-band photometry normalized by the modal quasar color as a function of redshift shows that this tail is well described by SMC-like reddening but not by reddening that is LMC-like, Galactic, or as recently described by Gaskell and coworkers. Extension to longer wavelengths using a subset of 1886 SDSS - Two Micron All Sky Survey matches confirms these results at high significance. We carry out Monte Carlo simulations that match the observed distribution of quasar spectral energy distributions using a Lorentzian dust-reddening distribution; 2% of quasars selected by the main SDSS targeting algorithm (i.e., that are not extincted out of the sample) have E(B-V) > 0.1, and less than 1% have E(B-V) > 0.2, where the extinction is relative to quasars with modal colors. Reddening is uncorrelated with the presence of intervening narrow-line absorption systems, but reddened quasars are much more likely to show narrow absorption at the redshift of the quasar than are unreddened quasars. Thus the reddening toward quasars is dominated by SMC-like dust at the quasar redshift.