Title: Dust in the reionization era: ALMA observations of a $z$=8.38 Galaxy,
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 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 $z\simeq8$. 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 ($\mu\simeq2$)
indicates A2744_YD4 has a stellar mass of $\sim$ 2$\times$10$^9$ M$_{\odot}$, a
star formation rate of $\sim20$ 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.