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Extremely red quasars from SDSS, BOSS and WISE: classification of optical spectra

Research output: Contribution to journalArticle

  • Nic Ross
  • Fred Hamann
  • Nadia L. Zakamska
  • Gordon T. Richards
  • Carolin Villforth
  • Michael A. Strauss
  • Jenny E. Greene
  • Rachael M. Alexandroff
  • W. Niel Brandt
  • Guilin Liu
  • Adam D. Myers
  • Isabelle Paris
  • Donald P. Schneider

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Original languageEnglish
JournalMonthly Notices of the Royal Astronomical Society
StatePublished - 14 Sep 2015


Quasars with extremely red infrared-to-optical colours are an interesting population that can test ideas about quasar evolution as well as orientation, obscuration and geometric effects in the so-called AGN unified model. To identify such a population, we match the quasar catalogues of the Sloan Digital Sky Survey (SDSS), the Baryon Oscillation Spectroscopic Survey (BOSS) to the Wide-Field Infrared Survey Explorer (WISE) to identify quasars with extremely high infrared-to-optical ratios. We identify 65 objects with rAB − W4Vega > 14 mag (i.e. Fν(22 μm)/Fν(r) ≳ 1000). This sample spans a redshift range of 0.28 < z < 4.36 and has a bimodal distribution, with peaks at z ∼ 0.8 and z ∼ 2.5. It includes three z > 2.6 objects that are detected in the W4 band but not W1 or W2 (i.e. ‘W1W2 dropouts'). The SDSS/BOSS spectra show that the majority of the objects are reddened type 1 quasars, type 2 quasars (both at low and high redshift) or objects with deep low-ionization broad absorption lines (BALs) that suppress the observed r-band flux. In addition, we identify a class of type 1 permitted broad emission-line objects at z ≃ 2–3 which are characterized by emission line rest-frame equivalent widths (REWs) of ≳150 Å, much larger than those of typical quasars. In particular, 55 per cent (45 per cent) of the non-BAL type 1s with measurable C iv in our sample have REW(C iv) > 100 (150) Å, compared to only 5.8 per cent (1.3 per cent) for non-BAL quasars in BOSS. These objects often also have unusual line ratios, such as very high N v/Ly α ratios. These large REWs might be caused by suppressed continuum emission analogous to type 2 quasars; however, there is no obvious mechanism in standard unified models to suppress the continuum without also obscuring the broad emission lines.

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