A large population of mid-infrared-selected, obscured active galaxies in the boötes field

R. C. Hickox, C. Jones, W. R. Forman, S. S. Murray, M. Brodwin, M. J.I. Brown, P. R. Eisenhardt, D. Stern, C. S. Kochanek, D. Eisenstein, R. J. Cool, B. T. Jannuzi, A. Dey, K. Brand, V. Gorjian, N. Caldwell

Research output: Contribution to journalArticlepeer-review

121 Scopus citations


We identify a population of 640 obscured and 839 unobscured AGNs at redshifts 0.7 < z ≲ 3 using multiwave-length observations of the 9 deg2 NOAO Deep Wide-Field Survey (NDWFS) region in Bootes. We select AGNs on the basis of Spitzer IRAC colors obtained by the IRAC Shallow Survey. Redshifts are obtained from optical spectroscopy or photometric redshift estimators. We classify the IR-selected AGNs as IRAGN 1 (unobscured) and IRAGN 2 (obscured) using a simple criterion based on the observed optical to mid-IR color, with a selection boundary of R - [4.5] = 6.1, where R and [4.5] are the Vega magnitudes in the R and IRAC 4.5 μm bands, respectively. We verify this selection using X-ray stacking analyses with data from the Chandra XBoötes survey, as well as optical photometry from NDWFS and spectroscopy from MMT/AGES. We show that (1) these sources are indeed AGNs, and (2) the optical/IR color selection separates obscured sources (with average NH ∼ 3 × 1022 cm-2 obtained from X-ray hardness ratios, and optical colors and morphologies typical of galaxies) and unobscured sources (with no X-ray absorption, and quasar colors and morphologies), with a reliability of a; 80%. The observed numbers of IRAGNs are comparable to predictions from previous X-ray, optical, and IR luminosity functions, for the given redshifts and IRAC flux limits. We observe a bimodal distribution in R - [4.5] color, suggesting that luminous IR-selected AGNs have either low or significant dust extinction, which may have implications for models of AGN obscuration.

Original languageEnglish (US)
Pages (from-to)1365-1387
Number of pages23
JournalAstrophysical Journal
Issue number2
StatePublished - Dec 20 2007


  • Galaxies: active
  • Infrared: galaxies
  • Quasars: general
  • Surveys
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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