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 (ALMA) detection of the [O III] 88 $\hat{I}¼$m line and rest-frame 90 $\hat{I}¼$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 $z=8.3118\pm 0.0003$, making this object one of the furthest galaxies ever identified spectroscopically. The observed 850 $\hat{I}¼$m flux density of $137\pm 26$ $\hat{I}¼$Jy corresponds to a de-lensed total infrared (IR) luminosity of $L_{\mathrm{IR}}=(1.7\pm 0.3)\times 10^{11}{L}_{\odot }$ if assuming a dust temperature of $T_{\mathrm{dust}}=50$ K and an emissivity index of ${\hat{I}}^2=1.5$, yielding a large dust mass of $4\times 10^6{M}_{\odot }$. The ultraviolet-to-far IR spectral energy distribution modeling where the [O III] emissivity model is incorporated suggests the presence of a young ($\ddot{I}{„}_{\mathrm{age}}\approx 4$ Myr), star-forming (SFR $\approx 60M_{\odot }$ yr${}^{-1}$), moderately metal-polluted ($Z\approx 0.2Z_{\odot }$) stellar component with a mass of $M_{\mathrm{star}}=3\times 10^8{M}_{\odot }$. An analytic dust mass evolution model with a single episode of star-formation does not reproduce the metallicity and dust mass in $\ddot{I}{„}_{\mathrm{age}}\approx 4$ Myr, suggesting a pre-existing evolved stellar component with $M_{\mathrm{star}}\sim 3\times 10^9{M}_{\odot }$ and $\ddot{I}{„}_{\mathrm{age}}\sim 0.3$ Gyr as the origin of the dust mass.

Accepted by ApJ. 18 pages, 10 figures, 5 tables