Mining for Dust in Type 1 Quasars

TL;DR

This study analyzes dust extinction in approximately 35,000 quasars across a wide redshift range, revealing that a small but significant fraction exhibit reddening consistent with SMC-like dust, and that intrinsic spectral properties vary with continuum color.

Contribution

It introduces a hierarchical Bayesian approach to quantify dust reddening distributions in quasars, linking spectral properties with dust and orientation effects, which is novel in large quasar samples.

Findings

2.5% of non-BAL quasars show E(B-V)>0.1

Quasars are intrinsically bluer than the mean composite

Redder quasars have narrower Balmer lines and stronger ionizing lines

Abstract

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 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 powerlaw 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 ionizing spectral lines, the latter indicating a harder continuum in the extreme-UV and the former pointing to differences in black hole mass and/or orientation.