Misaligned Discs as Obscurers in Active Galaxies
  We review critically the evidence concerning the fraction of Active Galactic
Nuclei (AGN) which appear as Type 2 AGN, carefully distinguishing strict Type 2
AGN from both more lightly reddened .Type 1 AGN, and from low excitation narrow
line AGN, which may represent a different mode of activity. Low excitation AGN
occur predominantly at low luminosities; after removing these, true Type 2 AGN
represent 58% of all AGN, and lightly reddened Type 1 AGN 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 AGN of exactly 50%. This 'tilted disc' 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.
