The Brightest Ly$\alpha$ Emitter: Pop III or Black Hole?
  CR7 is the brightest $z=6.6 \, {\rm Ly}\alpha$ emitter (LAE) known to date,
and spectroscopic follow-up by Sobral et al. (2015) suggests that CR7 might
host Population (Pop) III stars. We examine this interpretation using
cosmological hydrodynamical simulations. Several simulated galaxies show the
same "Pop III wave" pattern observed in CR7. However, to reproduce the extreme
CR7 ${\rm Ly}\alpha$/HeII1640 line luminosities ($L_{\rm \alpha/He II}$) a
top-heavy IMF and a massive ($>10^{7}{\rm M}_{\odot}$) PopIII burst with age
$<2$ Myr are required. Assuming that the observed properties of ${\rm
Ly}\alpha$ and HeII emission are typical for Pop III, we predict that in the
COSMOS/UDS/SA22 fields, 14 out of the 30 LAEs at $z=6.6$ with $L_{\alpha}
>10^{43.3}{\rm erg}\,{\rm s}^{-1}$ should also host Pop III stars producing an
observable $L_{\rm He II}>10^{42.7}{\rm erg}\,{\rm s}^{-1}$. As an alternate
explanation, we explore the possibility that CR7 is instead powered by
accretion onto a Direct Collapse Black Hole (DCBH). Our model predicts
$L_{\alpha}$, $L_{\rm He II}$, and X-ray luminosities that are in agreement
with the observations. In any case, the observed properties of CR7 indicate
that this galaxy is most likely powered by sources formed from pristine gas. We
propose that further X-ray observations can distinguish between the two above
scenarios.
