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

Abstract

How far can we see galaxies in the distant Universe? When are the first metals and the first dust formed?
We have now the first results on these topics (Laporte et al. 2017, ApJL, 837, 21L) based 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 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 lensing magnification indicates A2744_YD4
has a stellar mass of ∼2 × 10^9 solar mass, a star formation rate of ∼20 solar mass_yr^‑1 and a dust mass of ∼6 × 10^6 solar mass.
We discuss the implications of the formation of such a dust mass only ≃ 200 Myr after the onset of cosmic reionization.