The Origins of AGN Obscuration: The 'Torus' as a Dynamical, Unstable
  Driver of Accretion
  Multi-scale simulations have made it possible to follow gas inflows onto
massive black holes (BHs) from galactic scales to the accretion disk. When
sufficient gas is driven towards the BH, gravitational instabilities
generically form lopsided, eccentric disks that propagate inwards. The lopsided
stellar disk exerts a strong torque on the gas disk, driving inflows that fuel
rapid BH growth. Here, we investigate whether the same gas disk is the 'torus'
invoked to explain obscured AGN. The disk is generically thick and has
characteristic ~1-10 pc sizes and masses resembling those required of the
torus. The scale heights and obscured fractions of the predicted torii are
substantial even in the absence of strong stellar feedback providing the
vertical support. Rather, they can be maintained by strong bending modes and
warps excited by the inflow-generating instabilities. Other properties commonly
attributed to feedback processes may be explained by dynamical effects:
misalignment between torus and host galaxy, correlations between local SFR and
turbulent gas velocities, and dependence of obscured fractions on AGN
luminosity or SFR. We compare the predicted torus properties with observations
of gas surface density profiles, kinematics, scale heights, and SFR densities
in AGN nuclei, and find that they are consistent. We argue that it is not
possible to reproduce these observations and the observed column density (N_H)
distribution without a clumpy gas distribution, but allowing for clumping on
small scales the predicted N_H distribution is in good agreement with
observations from 10^20-27 cm^-2. We examine how N_H scales with galaxy and AGN
properties, and find that AGN feedback may be necessary to explain some trends
with luminosity and/or redshift. The torus is not merely a bystander or passive
fuel source for accretion, but is itself the mechanism driving accretion.
