Spectropolarimetric evidence for radiatively inefficient accretion in an optically dull active galaxy

Jonathan R. Trump, Tohru Nagao, Hiro Ikeda, Takashi Murayama, Christopher D Impey, John T. Stocke, Francesca Civano, Martin Elvis, Knud Jahnke, Brandon C. Kelly, Anton M. Koekemoer, Yoshi Taniguchi

Research output: Contribution to journalArticle

7 Citations (Scopus)

Abstract

We present Subaru/FOCAS spectropolarimetry of two active galaxies in the Cosmic Evolution Survey. These objects were selected to be optically dull, with the bright X-ray emission of an active galactic nucleus (AGN) but missing optical emission lines in our previous spectroscopy. Our new observations show that one target has very weak emission lines consistent with an optically dull AGN, while the other object has strong emission lines typical of a host-diluted Type 2 Seyfert galaxy. In neither source do we observe polarized emission lines, with 3σ upper limits of P BLR ≲ 2%. This means that the missing broad emission lines (and weaker narrow emission lines) are not due to simple anisotropic obscuration, e.g., by the canonical AGN torus. The weak-lined optically dull AGN exhibits a blue polarized continuum with P = 0.78% ± 0.07% at 4400 Å < λrest < 7200 Å (P = 1.37% ± 0.16% at 4400 Å < λrest < 5050Å). The wavelength dependence of this polarized flux is similar to that of an unobscured AGN continuum and represents the intrinsic AGN emission, either as synchrotron emission or the outer part of an accretion disk reflected by a clumpy dust scatterer. Because this intrinsic AGN emission lacks emission lines, this source is likely to have a radiatively inefficient accretion flow.

Original languageEnglish (US)
Article number23
JournalAstrophysical Journal
Volume732
Issue number1
DOIs
StatePublished - May 1 2011

Fingerprint

active galaxies
accretion
active galactic nuclei
continuums
Seyfert galaxies
occultation
accretion disks
light emission
synchrotrons
dust
spectroscopy

Keywords

  • accretion, accretion disks
  • galaxies: active
  • galaxies: nuclei
  • polarization
  • quasars: emission lines
  • radiation mechanisms: general

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Spectropolarimetric evidence for radiatively inefficient accretion in an optically dull active galaxy. / Trump, Jonathan R.; Nagao, Tohru; Ikeda, Hiro; Murayama, Takashi; Impey, Christopher D; Stocke, John T.; Civano, Francesca; Elvis, Martin; Jahnke, Knud; Kelly, Brandon C.; Koekemoer, Anton M.; Taniguchi, Yoshi.

In: Astrophysical Journal, Vol. 732, No. 1, 23, 01.05.2011.

Research output: Contribution to journalArticle

Trump, JR, Nagao, T, Ikeda, H, Murayama, T, Impey, CD, Stocke, JT, Civano, F, Elvis, M, Jahnke, K, Kelly, BC, Koekemoer, AM & Taniguchi, Y 2011, 'Spectropolarimetric evidence for radiatively inefficient accretion in an optically dull active galaxy', Astrophysical Journal, vol. 732, no. 1, 23. https://doi.org/10.1088/0004-637X/732/1/23
Trump, Jonathan R. ; Nagao, Tohru ; Ikeda, Hiro ; Murayama, Takashi ; Impey, Christopher D ; Stocke, John T. ; Civano, Francesca ; Elvis, Martin ; Jahnke, Knud ; Kelly, Brandon C. ; Koekemoer, Anton M. ; Taniguchi, Yoshi. / Spectropolarimetric evidence for radiatively inefficient accretion in an optically dull active galaxy. In: Astrophysical Journal. 2011 ; Vol. 732, No. 1.
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AB - We present Subaru/FOCAS spectropolarimetry of two active galaxies in the Cosmic Evolution Survey. These objects were selected to be optically dull, with the bright X-ray emission of an active galactic nucleus (AGN) but missing optical emission lines in our previous spectroscopy. Our new observations show that one target has very weak emission lines consistent with an optically dull AGN, while the other object has strong emission lines typical of a host-diluted Type 2 Seyfert galaxy. In neither source do we observe polarized emission lines, with 3σ upper limits of P BLR ≲ 2%. This means that the missing broad emission lines (and weaker narrow emission lines) are not due to simple anisotropic obscuration, e.g., by the canonical AGN torus. The weak-lined optically dull AGN exhibits a blue polarized continuum with P = 0.78% ± 0.07% at 4400 Å < λrest < 7200 Å (P = 1.37% ± 0.16% at 4400 Å < λrest < 5050Å). The wavelength dependence of this polarized flux is similar to that of an unobscured AGN continuum and represents the intrinsic AGN emission, either as synchrotron emission or the outer part of an accretion disk reflected by a clumpy dust scatterer. Because this intrinsic AGN emission lacks emission lines, this source is likely to have a radiatively inefficient accretion flow.

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