Assembly of supermassive black hole seeds
  We present a suite of six fully cosmological, three-dimensional simulations
of the collapse of an atomic cooling halo in the early Universe. We use the
moving-mesh code arepo with an improved primordial chemistry network to evolve
the hydrodynamical and chemical equations. The addition of a strong
Lyman-Werner background suppresses molecular hydrogen cooling and permits the
gas to evolve nearly isothermally at a temperature of about 8000 K. Strong
gravitational torques effectively remove angular momentum and lead to the
central collapse of gas, forming a supermassive protostar at the center of the
halo. We model the protostar using two methods: sink particles that grow
through mergers with other sink particles, and a stiff equation of state that
leads to the formation of an adiabatic core. We impose threshold densities of
$10^8$, $10^{10}$, and $10^{12}\,\text{cm}^{-3}$ for the sink particle
formation and the onset of the stiff equation of state to study the late,
intermediate, and early stages in the evolution of the protostar, respectively.
We follow its growth from masses $\simeq 10\,\text{M}_\odot$ to $\simeq
10^5\,\text{M}_\odot$, with an average accretion rate of
$\langle\dot{M}_\star\rangle \simeq 2\,\text{M}_\odot\,\text{yr}^{-1}$ for sink
particles, and $\simeq 0.8 - 1.4\,\text{M}_\odot\,\text{yr}^{-1}$ for the
adiabatic cores. At the end of the simulations, the HII region generated by
radiation from the central object has long detached from the protostellar
photosphere, but the ionizing radiation remains trapped in the inner host halo,
and has thus not yet escaped into the intergalactic medium. Fully coupled,
radiation-hydrodynamics simulations hold the key for further progress.
