No evidence for Population III stars or a Direct Collapse Black Hole in the z = 6.6 Lyman-$\alpha$ emitter 'CR7'

TL;DR

This study refutes the presence of Population III stars or a direct collapse black hole in the z=6.6 galaxy CR7, instead suggesting more conventional explanations like a low-mass AGN or a young starburst with specific metallicity and emission features.

Contribution

The paper provides new observational evidence that challenges previous exotic interpretations of CR7, favoring standard astrophysical models based on emission line analysis and updated infrared data.

Findings

CR7's IRAC color cannot be explained by Pop. III or DCBH models.

Strong [OIII] emission likely contaminates the IRAC 3.6 micron band.

A low-mass AGN or a young, low-metallicity starburst can explain the observed features.

Abstract

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).