Why X-ray-selected active galactic nuclei appear optically dull

J. R. Rigby, G. H. Rieke, J. L. Donley, A. Alonso-Herrero, P. G. Pérez-González

Research output: Contribution to journalArticlepeer-review

83 Scopus citations


We investigate why half of X-ray-selected active galactic nuclei (AGNs) in deep surveys lack signs of accretion in their optical spectra. The majority of these "optically dull" AGNs are no more than ×6 times fainter than their host galaxies in rest-frame R band; as such, AGN lines are unlikely to be overwhelmed by stellar continuum in at least half the sample. We find that optically dull AGNs have the mid-infrared emission and LX/L IR ratios characteristic of local Seyfert galaxies, suggesting that the cause of optical dullness is not missing UV-optical continua. We compare the morphologies of 22 optically dull and 9 optically active AGNs at 0.5 < z < 0.8 and find that optically dull AGNs show a wide range of axis ratio, but optically active AGNs have only very round axis ratios. We conclude that hard X-rays select AGNs in host galaxies with a wide range of inclination angle, but only those AGNs in the most face-on or spheroidal host galaxies show optical emission lines. Thus, extranuclear dust in the host galaxy plays an important role in hiding the emission lines of optically dull AGNs.

Original languageEnglish (US)
Pages (from-to)115-133
Number of pages19
JournalAstrophysical Journal
Issue number1 I
StatePublished - Jul 1 2006


  • Galaxies: active
  • Infrared: galaxies
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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