Evidence for a direct collapse black hole in the Lyman-alpha source CR7
  Throughout the epoch of reionization the most luminous Ly{\alpha} emitters
are capable of ionizing their own local bubbles. The CR7 galaxy at $z \approx
6.6$ stands out for its combination of exceptionally bright Ly{\alpha} and HeII
1640 Angstrom line emission but absence of metal lines. As a result CR7 may be
the first viable candidate host of a young primordial starburst or direct
collapse black hole. High-resolution spectroscopy reveals a +160 km s$^{-1}$
velocity offset between the Ly{\alpha} and HeII line peaks while the spatial
extent of the Ly{\alpha} emitting region is $\sim 16$ kpc. The observables are
indicative of an outflow signature produced by a strong central source. We
present one-dimensional radiation-hydrodynamics simulations incorporating
accurate Ly{\alpha} feedback and ionizing radiation to investigate the nature
of the CR7 source. We find that a Population III star cluster with $10^5$ K
blackbody emission ionizes its environment too efficiently to generate a
significant velocity offset. However, a massive black hole with a nonthermal
Compton-thick spectrum is able to reproduce the Ly{\alpha} signatures as a
result of higher photon trapping and longer potential lifetime. For both
sources, Ly{\alpha} radiation pressure turns out to be dynamically important.
