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Jonathan R. Trump et al. 2009 ApJ 706 797 doi:10.1088/0004-637X/706/1/797

THE NATURE OF OPTICALLY DULL ACTIVE GALACTIC NUCLEI IN COSMOS*

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Jonathan R. Trump1, Chris D. Impey1, Yoshi Taniguchi2, Marcella Brusa3, Francesca Civano4, Martin Elvis4, Jared M. Gabor1, Knud Jahnke5, Brandon C. Kelly4,12, Anton M. Koekemoer6, Tohru Nagao2, Mara Salvato7, Yasuhiro Shioya2, Peter Capak7, John P. Huchra4, Jeyhan S. Kartaltepe8, Giorgio Lanzuisi9, Patrick J. McCarthy10, Vincenzo Maineri11 and Nick Z. Scoville7

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Abstract References Cited By

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.


Footnote
*  Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA Inc, under NASA contract NAS 5-26555; the Magellan Telescope, which is operated by the Carnegie Observatories; and the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.
Keywords

accretion, accretion disks; black hole physics; galaxies: active; galaxies: nuclei; X-rays: galaxies


Dates

Issue 1 (2009 November 20)

Received 2009 August 1, accepted for publication 2009 October 13

Published 2009 November 5



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