2019ApJ...874...27T

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 L_IR=(1.7±0.3)×10¹¹L_☉ if assuming a dust temperature of T_dust = 50 K and an emissivity index of β = 1.5, yielding a large dust mass of 4×10⁶M_☉. 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 (SFR~60M_☉ yr^–1), moderately metal-polluted (Z ≃ 0.2Z_☉) stellar component with a mass of M_star = 3 x 10⁸ 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 x 10⁹ M_☉ and τ_age ∼ 0.3 Gyr as the origin of the dust mass.