On the nature of the luminous Lya emitter CR7 and its UV components: physical conditions and JWST predictions
2017
We present new HST/WFC3 grism observations and re-analyse VLT data to unveil the continuum, variability and rest-frame UV lines of the three UV components of the most luminous Ly-alpha (Lya) emitter at z=6.6, COSMOS Redshift 7 (CR7). Our re-reduced, flux calibrated X-SHOOTER spectra of CR7 reveal a tentative detection of HeII with F(HeII)=$(1.8\pm0.7)\times10^{-17}$erg s$^{-1}$cm$^{-2}$ and we identify the signal (~2.6$\sigma$) as coming only from observations obtained along the major axis of Lya emission. There is a change of +0.2-0.5mag in UltraVISTA J band data for CR7 from DR2 to DR3, which virtually eliminates the strong J-band excess previously interpreted as being caused by HeII. Our WFC3 grism spectra provide a significant detection of the UV continuum of CR7's clump A, yielding an excellent fit to a power law with $\beta=-2.4\pm0.4$ and $M_{UV}=-21.7\pm0.3$, consistent with no variability. HST grism data fail to detect any rest-frame UV line in clump A above 3$\sigma$, yielding F(HeII)<$0.5\times10^{-17}$erg s$^{-1}$cm$^{-2}$ (EW$_0$<10A) at a 95% confidence level. Clump C is tentatively identified as a potential variable and high ionisation source with F(HeII)=$(1.0\pm0.4)\times10^{-17}$erg s$^{-1}$cm$^{-2}$. We perform CLOUDY modelling to constrain the metallicity and the ionising nature of CR7, and also make emission-line predictions for JWST/NIRSpec. CR7 seems to be actively forming stars without any clear AGN activity in clumps A and B, consistent with a metallicity of ~0.05-0.2 Z$_{\odot}$ and with component A experiencing the most massive starburst. Component C may host a high ionisation source/AGN. Our results highlight the need for spatially resolved information to study the complex formation and assembly of early galaxies within the epoch of re-ionisation.
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