On the nature and physical conditions of the luminous Lya emitter CR7
  and its rest-frame UV components
  We present new HST/WFC3 observations and re-analyse VLT data to unveil the
continuum, variability and rest-frame UV lines of the multiple UV clumps of the
most luminous Ly$\alpha$ emitter at z=6.6, CR7. Our re-reduced, flux calibrated
X-SHOOTER spectra of CR7 reveal a HeII emission line in observations obtained
along the major axis of Lyman-alpha (Lya) emission with the best seeing
conditions. HeII is spatially offset by +0.8'' from the peak of Lya emission,
and it is found towards clump B. Our WFC3 grism spectra detects the UV
continuum of CR7's clump A, yielding a power law with
$\beta=-2.5^{+0.6}_{-0.7}$ and $M_{UV}=-21.87^{+0.25}_{-0.20}$. No significant
variability is found for any of the UV clumps on their own, but there is
tentative (~2.2$\sigma$) brightening of CR7 in F110W as a whole from 2012 to
2017. HST grism data fail to robustly detect rest-frame UV lines in any of the
clumps, implying fluxes <2x10$^{-17}$ erg s$^{-1}$ cm$^{-2}$ (3 $\sigma$). We
perform CLOUDY modelling to constrain the metallicity and the ionising nature
of CR7. CR7 seems to be actively forming stars without any clear AGN activity
in clump A, consistent with a metallicity of ~0.05-0.2 Z$_{\odot}$. Component C
or an inter-clump component between B and C may host a high ionisation source.
Our results highlight the need for spatially resolved information to study the
formation and assembly of early galaxies.
