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Between r ~ 1 kpc and the black hole's accretion disk proper (r ltapprox 103 GMh / c2 ~ 0.03 M8 pc for a black hole of mass Mh = 108 M8 Msun) lies the transition disk. Unlike the accretion disk, this disk will be heated primarily by external radiation rather than by the local release of gravitational binding energy. Beyond r ~ 20 sigma2-2 M8 pc the stars in the galaxy, of velocity dispersion 102 sigma2 km s-1 determine the potential. A photoionized or molecular disk will be self-gravitating and Jeans unstable if its column density exceeds

Equation 3 (3)

where h is the scale height of the disk and T = 103 T3 K the gas temperature. If this gas flows in on a timescale tin to supply the black hole with an accretion rate Mdot = dotm Msun yr-1, then Sigma ~ 1 dotm rpc-1 v300-1 (tin / torb)g cm-2 where the orbital speed of the gas is 300 v300 km s-1 and torb is the orbital time of gas at radius r. Purely local viscosity results in enormous inflow times, and Begelman, Frank and Shlosman (papers in these proceedings, and references therein) have argued that the angular momentum transport is determined by self-gravitation and global bar instabilities, so that the mean column density always exceeds Sigma sg. Dust in such a column has an enormous optical depth to UV photons: tau gtapprox 104 T31/2 sigma2 rpc-1. Provided the disk is warped (by any of the mechanisms described in section 2) or flared, infrared reradiation is inevitable. A disk warped through angle theta will intercept and reradiate a fraction ~ theta / 3 of the luminosity of the central source. For idealized dust of constant alpha, if C = d(covering factor) / dln r propto rq, then in the `inertial range' (kTmin << h<nu << kTmax) the superposition of dust emission from all radii produces a reradiated spectrum with Lnu propto nu-s, where

Equation 4 (4)

Since substantial warps are to be expected on all scales, we expect in some average sense q appeq 0 and hence s ~ 1. A diversity of bumps and wiggles is to be expected in individual objects, depending on the radii (and hence, via figure 1, wavelength) where their warps are most pronounced, the size and composition of their dust grains, and whether they are viewed from an angle where dust at a large radius absorbs re-emission from the interior. This seems in accord with the infrared spectral energy distributions of AGN: a great variety, with a median s ~ 1 (Sanders et al. 1989, 1988b). As discussed in section 2, the properties of more realistic galactic dust modify slightly the simple result of equation (4), and by flattening the T(r) relation introduces a propensity for 3-5 µm ``bumps'' (see figure 2).

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