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

Understand

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$_{0}\approx145$ A, is undetected in [CII] to a limit of L$_{\rm [CII]}<2.2\times10^7$ L$_{\odot}$ implying a metallicity $Z\lesssim0.07 Z_{\odot}$.
• Intriguingly, our HST data indicates a red UV slope $\beta=-1.1\pm0.7$, at odds with the low dust content.