2017ApJ...837L..21L

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 z≃8. Deep X-SHOOTER spectra confirms the high-redshift identity of A2744_YD4 via the detection of Lyα emission at a redshift z = 8.38. The association with the ALMA detection is confirmed by the presence of [O III] 88 µm emission at the same redshift. Although both emission features are only significant at the 4σ 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 (µ≃2) indicates A2744_YD4 has a stellar mass of ∼2 x 10⁹ M_☉, a star formation rate of ∼20 M_☉ yr^–1 and a dust mass of ∼6 x 10⁶ M_☉. We discuss the implications of the formation of such a dust mass only ≃200 Myr after the onset of cosmic reionization.