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

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

Research output: Contribution to journalArticlepeer-review

83 Scopus citations


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 CIV 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 NV/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.

Original languageEnglish (US)
Pages (from-to)3932-3952
Number of pages21
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Nov 11 2015

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Galaxies: evolution
  • Infrared: galaxies
  • Quasars: general
  • Surveys


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