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≃8. Deep X-SHOOTER spectra confirms the high
redshift identity of A2744_YD4 via the detection of Lyman α emission at
a redshift z=8.38. The association with the ALMA detection is confirmed by
the presence of [OIII] 88μm 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 HST images confirms the physical association. Analysis of the available
photometric data and the modest gravitational magnification (μ≃2)
indicates A2744_YD4 has a stellar mass of ∼ 2×109 M⊙, a
star formation rate of ∼20 M⊙/yr and a dust mass of
∼6×106 M⊙. We discuss the implications of the
formation of such a dust mass only ≃200 Myr after the onset of cosmic
reionisation.Comment: Accepted for publication in ApJ