The nature of the Lyman Alpha Emitter CR7: a persisting puzzle

Abstract

The peculiar emission properties of the $z\sim 6.6$ Ly$\alpha$ emitter CR7 have been initially interpreted with the presence of either a direct collapse black hole (DCBH) or a substantial mass of Pop III stars. Instead, updated photometric observations by Bowler et al. (2016) seem to suggest that CR7 is a more standard system. Here we confirm that the original DCBH hypothesis is consistent also with the new data. Using radiation-hydrodynamic simulations, we reproduce the new IR photometry with two models involving a Compton-thick DCBH of mass $\approx 7\times 10^6{\mathrm{M}}_{\odot }$ accreting (a) metal-free ($Z=0$) gas with column density $N_H=8\times 10^{25}\mathrm{c}{\mathrm{m}}^{-2}$, or (b) low-metallicity gas ($Z=5\times 10^{-3}{\mathrm{Z}}_{\odot }$) with $N_H=3\times 10^{24}\mathrm{c}{\mathrm{m}}^{-2}$. The best fit model reproduces the photometric data to within $1\sigma$. Such metals can be produced by weak star-forming activity occurring after the formation of the DCBH. The main contribution to the Spitzer/IRAC $3.6\mu \mathrm{m}$ photometric band in both models is due to HeI/HeII $\lambda 4714,4687$ emission lines, while the contribution of [OIII] $\lambda 4959,5007$ emission lines, if present, is sub-dominant. Spectroscopic observations with JWST will be required to ultimately clarify the nature of CR7.

Accepted for publication in MNRAS Letters

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