2015MNRAS.453.2465P

CR7 is the brightest z = 6.6Lyα emitter (LAE) known to date, and spectroscopic follow-up by Sobral et al. 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 Lyα/HeII1640 line luminosities (L_α/He II) a top-heavy initial mass function and a massive ( ≳ 10⁷M_☉) Pop III burst with age ≲ 2 Myr are required. Assuming that the observed properties of Lyα 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 Lα > 10^43.3erg/s should also host Pop III stars producing an observable L_He II\gtrsim 10^42.7 erg s^-1. As an alternate explanation, we explore the possibility that CR7 is instead powered by accretion on to a direct collapse black hole. Our model predicts Lα, L_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.