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Published November 20, 2009 | Published
Journal Article Open

The Nature of Optically Dull Active Galactic Nuclei in COSMOS


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 f_X /f_O 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.

Additional Information

© 2009 American Astronomical Society. Print publication: Issue 1 (2009 November 20); received 2009 August 1; accepted for publication 2009 October 13; published 2009 November 5. J.R.T. acknowledges support from NSF ADP grant NNX08AJ28G, an ARCS fellowship, and an NSF-EAPSI/JSPS summer research fellowship. B.C.K. acknowledges support from NASA through the Hubble Fellowship grant HF-01220.01 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. We thank J. Rigby for comments on the axis ratios of AGN host galaxies, and A. Diamond-Stanic for help regarding line luminosity measurements.We additionally thank the anonymous referee for several suggestions which improved the quality of this paper.

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