Abstract 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/submillimeter Array (ALMA) indicate the proximate detection of a significant 1 mm continuum flux suggesting the presence of dust for a star-forming galaxy with a photometric redshift of <?CDATA $z\simeq 8$?> . Deep X-SHOOTER spectra confirms the high-redshift identity of A2744_YD4 via the detection of Ly α emission at a redshift z = 8.38. The association with the ALMA detection is confirmed by the presence of [O iii ] 88 μ m emission at the same redshift. Although both emission features are only significant at the 4 σ level, we argue their joint detection and the positional coincidence with a high-redshift dropout in the Hubble Space Telescope images confirms the physical association. Analysis of the available photometric data and the modest gravitational magnification ( <?CDATA $\mu \simeq 2$?> ) indicates A2744_YD4 has a stellar mass of ∼2 × 10 9 <?CDATA ${M}_{\odot }$?> , a star formation rate of ∼20 <?CDATA ${M}_{\odot }$?> yr −1 and a dust mass of ∼6 × 10 6 <?CDATA ${M}_{\odot }$?> . We discuss the implications of the formation of such a dust mass only <?CDATA $\simeq 200\,\mathrm{Myr}$?> after the onset of cosmic reionization.

Dust in the Reionization Era: ALMA Observations of a z = 8.38 Gravitationally Lensed Galaxy