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

Abstract

We present the Atacama Large Millimeter/submillimeter Array detection of the [O iii ] 88 μ m line and rest-frame 90 μ m 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 μ m flux density of 137 ± 26 μ Jy 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  ≈ 0.2 Z ⊙ ) stellar component with a mass of M star  = 3 × 10 8 M ⊙ . 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.