Dust in the reionization era: ALMA observations of a $z$=8.38 Galaxy

We report on the detailed analysis of a gravitationally-lensed Y-band dropout, A2744_YD4, selected from deep Hubble Space Telescope imaging in the Frontier Field cluster Abell 2744. Band 7 observations with the Atacama Large Millimeter Array (ALMA) indicate the proximate detection of a significant 1mm continuum flux suggesting the presence of dust for a star-forming galaxy with a photometric redshift of $z\simeq 8$. Deep X-SHOOTER spectra confirms the high redshift identity of A2744_YD4 via the detection of Lyman $\alpha$ emission at a redshift $z$=8.38. The association with the ALMA detection is confirmed by the presence of [OIII] 88$\mu$m emission at the same redshift. Although both emission features are only significant at the 4 $\sigma$ level, we argue their joint detection and the positional coincidence with a high redshift dropout in the HST images confirms the physical association. Analysis of the available photometric data and the modest gravitational magnification ($\mu \simeq 2$) indicates A2744_YD4 has a stellar mass of $\sim$ 2$\times$10${}^9$ M${}_{\odot }$, a star formation rate of $\sim 20$ M${}_{\odot }$/yr and a dust mass of $\sim$6$\times$10${}^6$ M${}_{\odot }$. We discuss the implications of the formation of such a dust mass only $\simeq$200 Myr after the onset of cosmic reionisation.