Evidence for a direct collapse black hole in the Lyman-alpha source CR7

TL;DR

This study investigates the nature of the CR7 galaxy, suggesting it is likely hosting a direct collapse black hole rather than a Population III star cluster, based on detailed radiation-hydrodynamics simulations and observed spectral features.

Contribution

The paper provides the first detailed radiation-hydrodynamics simulations that distinguish between a Population III star cluster and a direct collapse black hole as the source of CR7's unique emission features.

Findings

A Population III star cluster cannot produce the observed velocity offset.

A massive black hole with a nonthermal spectrum can reproduce the spectral signatures.

Ly{ extalpha} radiation pressure plays a significant role in the dynamics.

Abstract

Throughout the epoch of reionization the most luminous Ly{\alpha} emitters are capable of ionizing their own local bubbles. The CR7 galaxy at $z \approx 6.6$ stands out for its combination of exceptionally bright Ly{\alpha} and HeII 1640 Angstrom line emission but absence of metal lines. As a result CR7 may be the first viable candidate host of a young primordial starburst or direct collapse black hole. High-resolution spectroscopy reveals a +160 km s$^{-1}$ velocity offset between the Ly{\alpha} and HeII line peaks while the spatial extent of the Ly{\alpha} emitting region is $\sim 16$ kpc. The observables are indicative of an outflow signature produced by a strong central source. We present one-dimensional radiation-hydrodynamics simulations incorporating accurate Ly{\alpha} feedback and ionizing radiation to investigate the nature of the CR7 source. We find that a Population III star cluster with $10^5$ K blackbody emission ionizes its environment too efficiently to generate a significant velocity offset. However, a massive black hole with a nonthermal Compton-thick spectrum is able to reproduce the Ly{\alpha} signatures as a result of higher photon trapping and longer potential lifetime. For both sources, Ly{\alpha} radiation pressure turns out to be dynamically important.