The discovery of rest-frame UV colour gradients and a diversity of dust
  morphologies in bright z ~ 7 Lyman-break galaxies
  We present deep ALMA dust continuum observations for a sample of luminous
($M_{\rm UV} < -22$) star-forming galaxies at $z \simeq 7$. We detect five of
the six sources in the far-infrared (FIR), providing key constraints on the
obscured star-formation rate (SFR) and the infrared-excess-$\beta$
(IRX-$\beta$) relation without the need for stacking. Despite the galaxies
showing blue rest-frame UV slopes ($\beta \simeq -2$) we find that 35-75
percent of the total SFR is obscured. We find the IRX-$\beta$ relation derived
for these $z \simeq 7$ sources is consistent with that found for local
star-burst galaxies. Using our relatively high-resolution (FWHM $\simeq
0.7\,{\rm arcsec}$) observations we identify a diversity of dust morphologies
in the sample. We find both compact emission that appears offset relative to
the unobscured components and extended dust emission that is co-spatial with
the rest-frame UV light. In the majority of the sources we detect strong
rest-frame UV colour gradients (with up to $\Delta \beta \simeq 0.7$-$1.4$) as
probed by the multi-band UltraVISTA ground-based data. The observed redder
colours are spatially correlated with the location of the FIR detection. Our
results show that even in bright Lyman-break galaxies at $z \simeq 7$ the peak
of the star-formation is typically hosted by the fainter, redder, regions in
the rest-frame UV, which have an obscured fraction of $f_{\rm obs} \ge 0.8$. As
well as demonstrating the importance of dust obscured star-formation within the
Epoch of Reionization, these observations provide an exciting taster of the
rich spatially resolved datasets that will be obtained from JWST and
high-resolution ALMA follow-up at these redshifts.
