No evidence for Population III stars or a Direct Collapse Black Hole in
  the z = 6.6 Lyman-$\alpha$ emitter 'CR7'
  The z = 6.6 Lyman-$\alpha$ emitter 'CR7' has been claimed to have a
Population 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/IRAC detections for the main
component of CR7 at 3.6 and 4.5 microns, and show that it has a blue colour
([3.6] - [4.5] $= -1.2\pm 0.3$). 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 [OIII]4959,5007 emission line with an implied
rest-frame equivalent width of EW_0 (H$\beta$ + [OIII]) $\gtrsim 2000$\AA.
Furthermore, we find that new near-infrared data from the UltraVISTA survey
supports a weaker He II 1640 emission line than previously measured, with EW_0
$= 40 \pm 30$\AA. 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 [OIII] 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 AGN can reproduce the
observed features of CR7, including the lack of radio and X-ray detections.
Alternatively, a young, low-metallicity (~1/200 solar) starburst, modelled
including binary stellar pathways, can reproduce the inferred strength of the
He II line and simultaneously the strength of the observed [OIII] emission, but
only if the gas shows super-solar $\alpha$-element abundances (O/Fe ~ 5 O/Fe
solar).
