The growth of supermassive black holes fed by accretion disks
  Supermassive black holes are probably present in the centre of the majority
of the galaxies. There is a consensus that these exotic objects are formed by
the growth of seeds either by accreting mass from a circumnuclear disk and/or
by coalescences during merger episodes.
  The mass fraction of the disk captured by the central object and the related
timescale are still open questions, as well as how these quantities depend on
parameters like the initial mass of the disk or the seed or on the angular
momentum transport mechanism. This paper is addressed to these particular
aspects of the accretion disk evolution and of the growth of seeds.
  The time-dependent hydrodynamic equations were solved numerically for an
axi-symmetric disk in which the gravitational potential includes contributions
both from the central object and from the disk itself. The numerical code is
based on a Eulerian formalism, using a finite difference method of
second-order, according to the Van Leer upwind algorithm on a staggered mesh.
  The present simulations indicate that seeds capture about a half of the
initial disk mass, a result weakly dependent on model parameters. The
timescales required for accreting 50% of the disk mass are in the range 130-540
Myr, depending on the adopted parameters. These timescales permit to explain
the presence of bright quasars at z ~ 6.5. Moreover, at the end of the disk
evolution, a "torus-like" geometry develops, offering a natural explanation for
the presence of these structures in the central regions of AGNs, representing
an additional support to the unified model.
