Dust in the Reionization Era: ALMA Observations of a z=8.38 Gravitationally Lensed 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/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 similar or equal to 8. Deep X-SHOOTER spectra confirms the high-redshift identity of A2744_YD4 via the detection of Ly alpha emission at a redshift z = 8.38. The association with the ALMA detection is confirmed by the presence of [O III] 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 Hubble Space Telescope images confirms the physical association. Analysis of the available photometric data and the modest gravitational magnification (mu similar or equal to 2) indicates A2744_YD4 has a stellar mass of similar to 2. x. 10(9) M-circle dot a star formation rate of similar to 20 similar or equal to 200 Myr-1 and a dust mass of similar to 6. x. 10(6) M-circle dot We discuss the implications of the formation of such a dust mass only similar or equal to 200 Myr after the onset of cosmic reionization.