No evidence for Population III stars or a direct collapse black hole in the z = 6.6 Lyman α emitter 'CR7'

Abstract

The z = 6.6 Lyman α emitter 'CR7' has been claimed to have a Population III (Pop III) like stellar population, or alternatively, be a candidate direct collapse black hole (DCBH). In this paper, we investigate the evidence for these exotic scenarios using recently available, deeper, optical, near-infrared and mid-infrared imaging. We find strong Spitzer/Infrared Array Camera detections for the main component of CR7 at 3.6 and 4.5 μm, and show that it has a blue colour ([3.6] − [4.5] = −1.2 ± 0.3). This colour cannot be reproduced by current Pop III or pristine DCBH models. Instead, the results suggest that the [3.6] band is contaminated by the [O III]λλ 4959, 5007 emission line with an implied rest-frame equivalent width of EW0(H β + [O III]) ≳ 2000 Å. Furthermore, we find that new near-infrared data from the UltraVISTA survey supports a weaker He II λ1640 emission line than previously measured, with EW0 = 40 ± 30 Å. For the fainter components of CR7 visible in Hubble Space Telescope imaging, we find no evidence that they are particularly red as previously claimed, and show that the derived masses and ages are considerably uncertain. In light of the likely detection of strong [O III] emission in CR7, we discuss other more standard interpretations of the system that are consistent with the data. We find that a low-mass, narrow-line active galactic nucleus can reproduce the observed features of CR7, including the lack of radio and X-ray detections. Alternatively, a young, low-metallicity (∼1/200 Z⊙) starburst, modelled including binary stellar pathways, can reproduce the inferred strength of the He II line and simultaneously the strength of the observed [O III] emission, but only if the gas shows supersolar α-element abundances (O/Fe ≃ 5 (O/Fe)⊙).

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