Dust formation in AGN winds
  Infrared observations of active galactic nucleus (AGN) reveal emission from
the putative dusty circumnuclear 'torus' invoked by AGN unification, that is
heated up by radiation from the central accreting black hole (BH). The strong
9.7 and 18 micron silicate features observed in the AGN spectra both in
emission and absorption, further indicate the presence of such dusty
environments. We present detailed calculations of the chemistry of silicate
dust formation in AGN accretion disk winds. The winds considered herein are
magnetohydrodynamic (MHD) winds driven off the entire accretion disk domain
that extends from the BH vicinity to the radius of BH influence, of order of 1
to 100 pc depending on the mass of the resident BH. Our results indicate that
these winds provide conditions conducive to the formation of significant
amounts of dust, especially for objects accreting close to their Eddington
limit, making AGN a significant source of dust in the universe, especially for
luminous quasars. Our models justify the importance of a r to the power -1
density law in the winds for efficient formation and survival of dust grains.
The dust production rate scales linearly with the mass of the central BH and
varies as a power law of index between 2 to 2.5 with the dimensionless mass
accretion rate. The resultant distribution of the dense dusty gas resembles a
toroidal shape, with high column density and optical depths along the
equatorial viewing angles, in agreement with the AGN unification picture.
