A. Pallottini, P. Ferrara, F. Pacucci, S. Gallerani, S. Salvadori, R. Schneider, D. Schaerer, D. Sobral, J. Matthee

Abstract
CR7 is the brightest z = 6.6 Ly α 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 Ly α/He ii1640 line luminosities (Lα/HeII ) a top-heavy initial mass function and a massive ( ≳ 10⁷ M_⊙) Pop III burst with age ≲ 2 Myr are required. Assuming that the observed properties of Ly α and He ii 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_α > 10^43.3 erg s⁻¹ should also host Pop III stars producing an observable LHeII≳1042.7ergs−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 L_α, LHeII , 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.

Keywords
black hole physics - stars: Population III - galaxies: high-redshift

Monthly Notices of the Royal Astronomical Society
Volume 453, Issue 3, Page 2465
2015 November

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