The nature of the Lyman α emitter CR7: a persisting puzzle

Abstract

Abstract The peculiar emission properties of the z ∼ 6.6 Lyα emitter COSMOS redshift 7 (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. 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 infrared photometry with two models involving a Compton-thick DCBH of mass ≈7 × 106 M⊙ accreting (a) metal-free (Z = 0) gas with column density NH = 8 × 1025 cm−2 or (b) low-metallicity gas (Z = 5 × 10−3 Z⊙) with NH = 3 × 1024 cm−2. The best-fitting model reproduces the photometric data to within 1σ. 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 μm photometric band in both models is due to He i/He ii λλ4714, 4687 emission lines, while the contribution of [O iii] λλ4959, 5007 emission lines, if present, is sub-dominant. Spectroscopic observations with JWST will be required to ultimately clarify the nature of CR7.