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 $\approx {10}^{-7}\mathrm{M}\mathrm{p}{\mathrm{c}}^{-3}$ 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.