Misaligned Disks as Obscurers in Active Galaxies

Andy Lawrence, Martin Elvis

Research output: Contribution to journalArticlepeer-review

Abstract

We critically review the evidence concerning the fraction of active galactic nuclei (AGNs) that appear as Type 2 AGNs, carefully distinguishing strict Type 2 AGNs from both more lightly reddened Type 1 AGNs, and from low excitation narrow line AGNs, which may represent a different mode of activity. Low-excitation AGNs occur predominantly at low luminosities; after removing these, true Type 2 AGNs represent 58% ± 5% of all AGNs, and lightly reddened Type 1 AGNs a further ~15%. Radio, IR, and volume-limited samples all agree in showing no change of Type 2 fraction with luminosity. X-ray samples do show a change with luminosity; we discuss possible reasons for this discrepancy. We test a very simple picture which produces this Type 2 fraction with minimal assumptions. In this picture, infall from large scales occurs in random directions, but must eventually align with the inner accretion flow, producing a severely warped disk on parsec scales. If the re-alignment is dominated by tilt, with minimal twist, a wide range of covering factors is predicted in individual objects, but with an expected mean fraction of Type 2 AGNs of exactly 50%. This "tilted disk" picture predicts reasonable alignment of observed nuclear structures on average, but with distinct misalignments in individual cases. Initial case studies of the few well-resolved objects show that such misalignments are indeed present.

Original languageEnglish
Pages (from-to)561-570
Number of pages10
JournalAstrophysical Journal
Volume714
Issue number1
DOIs
Publication statusPublished - 1 May 2010

Keywords

  • accretion, accretion disks
  • galaxies: active
  • galaxies: nuclei
  • quasars: general
  • NARROW-LINE REGION
  • ABSORBING COLUMN DENSITIES
  • RAY SPECTRAL PROPERTIES
  • SEYFERT 2 GALAXIES
  • SWIFT BAT SURVEY
  • GALACTIC NUCLEI
  • RADIO GALAXIES
  • UNIFIED SCHEMES
  • MOLECULAR GAS
  • BLACK-HOLE

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