Formation of Massive Population III Galaxies through Photoionization
  Feedback: A Possible Explanation for CR7
  We explore the formation of massive high-redshift Population III (Pop III)
galaxies through photoionization feedback. We consider dark matter halos formed
from progenitors that have undergone no star formation as a result of early
reionization and photoevaporation caused by a nearby galaxy. Once such a halo
reaches $\approx 10^9~M_\odot$, corresponding to the Jeans mass of the
photoheated intergalactic medium (IGM) at $z\approx 7$, pristine gas is able to
collapse into the halo, potentially producing a massive Pop III starburst. We
suggest that this scenario may explain the recent observation of strong He II
1640~\AA~line emission in CR7, which is consistent with $\sim 10^7~M_\odot$ of
young Pop III stars. Such a large mass of Pop III stars is unlikely without the
photoionization feedback scenario, because star formation is expected to inject
metals into halos above the atomic cooling threshold ($\sim 10^8~M_\odot$ at $z
\approx 7$). We use merger trees to analytically estimate the abundance of
observable Pop III galaxies formed through this channel, and find a number
density of $\approx 10^{-7}~{\rm Mpc^{-3}}$ at $z=6.6$ (the redshift of CR7).
This is approximately a factor of ten lower than the density of Ly$\alpha$
emitters as bright as CR7.
