Mining for dust in type 1 quasars

Coleman M. Krawczyk, Gordon T. Richards, S. C. Gallagher, Karen M. Leighly, Paul C. Hewett, Nicholas P. Ross, P. B. Hall

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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 (αλ) 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.

Original languageEnglish
Article number203
JournalAstronomical Journal
Issue number6
Publication statusPublished - 1 Jun 2015


  • dust, extinction
  • infrared: galaxies
  • methods: statistical
  • quasars: absorption lines
  • quasars: emission lines
  • quasars: general


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