Exploring the nature of the Lyman-$\alpha$ emitter CR7

TL;DR

This study investigates the origins of the luminous high-redshift Lyman-$ extalpha$ emitter CR7, finding that direct collapse black holes are the most plausible source given current models, while primordial stars and remnants are unlikely.

Contribution

It introduces a semianalytical model to explore formation scenarios of CR7, ruling out primordial stars and stellar-mass black holes, and supporting direct collapse black holes as the likely origin.

Findings

Primordial stars cannot explain CR7 due to short lifetimes and early metal enrichment.

Stellar-mass black hole remnants are too small or metal-polluted to account for CR7.

Direct collapse black holes in metal-free haloes are the most plausible explanation.

Abstract

CR7 is the brightest Lyman-$\alpha$ emitter observed at $z>6$, which shows very strong Lyman-$\alpha$ and HeII 1640\AA\ line luminosities, but no metal line emission. Previous studies suggest that CR7 hosts either young primordial stars with a total stellar mass of $\sim 10^7\,\mathrm{M}_\odot$ or a black hole of $\gtrsim 10^6\,\mathrm{M}_\odot$. Here, we explore different formation scenarios for CR7 with a semianalytical model, based on the random sampling of dark matter merger trees. We are unable to reproduce the observational constraints with a primordial stellar source, given our model assumptions, due to the short stellar lifetimes and the early metal enrichment. Black holes that are the remnants of the first stars are either not massive enough, or reside in metal-polluted haloes, ruling out this possible explanation of CR7. Our models instead suggest that direct collapse black holes, which form in metal-free haloes exposed to large Lyman-Werner fluxes, are more likely the origin of CR7. However, this result is derived under optimistic assumptions and future observations are necessary to further constrain the nature of CR7.