Journal
ASTROPHYSICAL JOURNAL
Volume 714, Issue 1, Pages 561-570Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/714/1/561
Keywords
accretion, accretion disks; galaxies: active; galaxies: nuclei; quasars: general
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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 similar to 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.
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