Supermassive Black Hole Formation at High Redshifts via Direct Collapse
  in a Cosmological Context
  We study the early stage of the formation of seed supermassive black holes
via direct collapse in dark matter (DM) halos, in the cosmological context. We
perform high-resolution zoom-in simulations of such collapse at high-$z$. Using
the adaptive mesh refinement code ENZO, we resolve the formation and growth of
a DM halo, until its virial temperature reaches $\sim 10^4$K, atomic cooling
turns on, and collapse ensues. We demonstrate that direct collapse proceeds in
two stages, although they are not well separated. The first stage is triggered
by the onset of atomic cooling, and leads to rapidly increasing accretion rate
with radius, from $\dot M\sim 0.1\,M_\odot {\rm yr^{-1}}$ at the halo virial
radius to few $M_\odot \,{\rm yr^{-1}}$, around the scale radius $R_{\rm s}\sim
30$pc of the NFW DM density profile. The second stage of the collapse commences
when the gas density takes precedence over the DM density. This is associated
with the gas decoupling from the DM gravitational potential. The ensuing
collapse approximates that of an isothermal sphere with $\dot M ( r )\sim
$const. We confirm that the gas loses its angular momentum through
non-axisymmetric perturbations and gravitational torques, to overcome the
centrifugal barrier. During the course of the collapse, this angular momentum
transfer process happens on nearly all spatial scales, and the angular momentum
vector of the gas varies with position and time. Collapsing gas also exhibits
supersonic turbulent motions which suppress gas fragmentation, and are
characterized by density PDF consisting of a lognormal part and a high-density
power law tail.
