Detection of the Far-infrared [O III] and Dust Emission in a Galaxy at Redshift 8.312: Early Metal Enrichment in the Heart of the Reionization Era

We present the Atacama Large Millimeter/submillimeter Array detection of the [O iii] 88 μm line and rest-frame 90 μm dust continuum emission in a Y-dropout Lyman break galaxy (LBG), MACS0416-Y1 lying behind the Frontier Field cluster MACS J0416.1-2403. This [O iii] detection confirms the LBG with a spectroscopic redshift of z = 8.3118 ± 0.0003, making this object one of the farthest galaxies ever identified spectroscopically. The observed 850 μm flux density of 137 ± 26 μJy corresponds to a de-lensed total infrared (IR) luminosity of if assuming a dust temperature of T _dust = 50 K and an emissivity index of β = 1.5, yielding a large dust mass of . The ultraviolet-to-far-IR spectral energy distribution modeling where the [O iii] emissivity model is incorporated suggests the presence of a young (τ _age ≈ 4 Myr), star-forming ( yr ^-1 ), moderately metal-polluted (Z ≈ 0.2Z _o ) stellar component with a mass of M _star = 3 ×10 ⁸ M _o . An analytic dust mass evolution model with a single episode of star formation does not reproduce the metallicity and dust mass in τ _age ≈ 4 Myr, suggesting a pre-existing evolved stellar component with M _star ∼ 3 ×10 ⁹ M and τ _age ∼ 0.3 Gyr as the origin of the dust mass.