Direct Collapse to Supermassive Black Hole Seeds with Radiative
  Transfer: Isolated Halos
  Direct collapse within dark matter (DM) halos is a promising path to form
supermassive black hole (SMBH) seeds at high redshifts. The outer part of this
collapse remains optically thin, and has been studied intensively using
numerical simulations. However, the innermost region of the collapse is
expected to become optically thick and requires us to follow the radiation
field in order to understand its subsequent evolution. So far, the adiabatic
approximation has been used exclusively for this purpose. We apply radiative
transfer in the flux-limited diffusion (FLD) approximation to solve the
evolution of coupled gas and radiation, for isolated halos. For direct collapse
within isolated DM halos, we find that (1) the photosphere forms at ~10^{-6} pc
and rapidly expands outward. (2) A central core forms, with a mass of ~1 Mo,
supported by thermal gas pressure gradients and rotation. (3) Growing thermal
gas and radiation pressure gradients dissolve it. (4) This process is
associated with a strong anisotropic outflow, and another core forms nearby and
grows rapidly. (5) Typical radiation luminosity emerging from the photosphere
encompassing these cores is ~5 x 10^{37}-5 x 10^{38} erg/s, of order the
Eddington luminosity. (6) Two variability timescales are associated with this
process: a long one, which is related to the accretion flow within the central
~10^{-4}-10^{-3} pc, and ~0.1 yr, which is related to radiation diffusion. (7)
Adiabatic models have been run for comparison and their evolution differs
profoundly from that of the FLD models, by forming a central
geometrically-thick disk. Overall, an adiabatic equation of state is not a good
approximation to the advanced stage of direct collapse, mainly because the
radiation in the FLD is capable of escaping due to anisotropy in the optical
depth and associated gradients.
