Mining for dust in type 1 quasars

We explore the extinction/reddening of ∼35,000 uniformly selected quasars with $0<z⩽5.3$ in order to better understand their intrinsic optical/ultraviolet (UV) spectral energy distributions. Using rest-frame optical–UV photometry taken from the Sloan Digital Sky Survey's (SDSS) 7th data release, cross-matched to WISE in the mid-infrared, 2MASS and UKIDSS in the near-infrared, and GALEX in the UV, we isolate outliers in the color distribution and find them well described by an SMC-like reddening law. A hierarchical Bayesian model with a Markov Chain Monte Carlo sampling method was used to find distributions of power law indices and $E(B-V)$ consistent with both the broad absorption line (BAL) and non-BAL samples. We find that, of the ugriz color-selected type 1 quasars in SDSS, 2.5% (13%) of the non-BAL (BAL) sample are consistent with $E(B-V)>0.1$ and 0.1% (1.3%) with $E(B-V)>0.2$. Simulations show both populations of quasars are intrinsically bluer than the mean composite, with a mean spectral index (${\alpha }_{\lambda }$) of −1.79 (−1.83). The emission and absorption-line properties of both samples reveal that quasars with intrinsically red continua have narrower Balmer lines and stronger high-ionization emission lines, the latter indicating a harder continuum in the extreme-UV and the former pointing to differences in black hole mass and/or orientation.