The nature of optically dull active galactic nuclei in cosmos

Jonathan R. Trump, Chris D. Impey, Yoshi Taniguchi, Marcella Brusa, Francesca Civano, Martin Elvis, Jared M. Gabor, Knud Jahnke, Brandon C. Kelly, Anton M. Koekemoer, Tohru Nagao, Mara Salvato, Yasuhiro Shioya, Peter Capak, John P. Huchra, Jeyhan S. Kartaltepe, Giorgio Lanzuisi, Patrick J. McCarthy, Vincenzo Maineri, Nick Z. Scoville

Research output: Contribution to journalArticlepeer-review

50 Scopus citations


We present infrared, optical, and X-ray data of 48 X-ray bright, optically dull active galactic nuclei (AGNs) in the COSMOS field. These objects exhibit the X-ray luminosity of an AGN but lack broad and narrow emission lines in their optical spectrum. We show that despite the lack of optical emission lines, most of these optically dull AGNs are not well described by a typical passive red galaxy spectrum: instead they exhibit weak but significant blue emission like an unobscured AGN. Photometric observations over several years additionally show significant variability in the blue emission of four optically dull AGNs. The nature of the blue and infrared emission suggest that the optically inactive appearance of these AGNs cannot be caused by obscuration intrinsic to the AGNs. Instead, up to ∼70% of optically dull AGNs are diluted by their hosts, with bright or simply edge-on hosts lying preferentially within the spectroscopic aperture. The remaining ∼30% of optically dull AGNs have anomalously high fX /fO ratios and are intrinsically weak, not obscured, in the optical. These optically dull AGNs are best described as a weakly accreting AGN with a truncated accretion disk from a radiatively inefficient accretion flow.

Original languageEnglish (US)
Pages (from-to)797-809
Number of pages13
JournalAstrophysical Journal
Issue number1
StatePublished - 2009


  • Accretion, accretion disks
  • Black hole physics
  • Galaxies: Active
  • Galaxies: Nuclei
  • X-rays: Galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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