Resolved UV and [CII] structures of luminous galaxies within the epoch of reionisation

We present new deep ALMA and HST/WFC3 observations of MASOSA and VR7, two luminous Ly$\alpha$ emitters (LAEs) at $z=6.5$, for which the UV continuum level differ by a factor four. No IR dust continuum emission is detected in either, indicating little amounts of obscured star formation and/or high dust temperatures. MASOSA, with a UV luminosity M${}_{1500}=-20.9$, compact size and very high Ly$\alpha$ EW${}_0\approx 145$ A, is undetected in [CII] to a limit of L${}_{[\mathrm{C}\mathrm{I}\mathrm{I}]}<2.2\times {10}^7$ L${}_{\odot }$ implying a metallicity $Z\lesssim0.07 Z_{\odot}$\mathrm{.}Intriguingly,ourHSTdataindicatesaredUVslope$\beta=-1.1\pm0.7$, at odds with the low dust content. VR7, which is a bright (M${}_{1500}=-22.4$) galaxy with moderate color ($\beta =-1.4\pm 0.3$) and Ly$\alpha$ EW${}_0=34$ A, is clearly detected in [CII] emission (S/N=15). VR7’s rest-frame UV morphology can be described by two components separated by $\approx 1.5$ kpc and is globally more compact than the [CII] emission. The global [CII]-UV ratio indicates $Z\approx 0.2Z_{\odot }$, but there are large variations in the UV-[CII] ratio on kpc scales. We also identify diffuse, possibly outflowing, [CII]-emitting gas at $\approx 100$ km s${}^{-1}$ with respect to the peak. VR7 appears assembling its components at a slightly more evolved stage than other luminous LAEs, with outflows already shaping its direct environment at $z\sim 7$. Our results further indicate that the global [CII]-UV relation steepens at SFR $<30$ M${}_{\odot }$ yr${}^{-1}$, naturally explaining why the [CII]-UV ratio is anti-correlated with Ly$\alpha$ EW in many, but not all, observed LAEs.