JWST MIRI Detections of Hα and O iii and a Direct Metallicity Measurement of the z = 10.17 Lensed Galaxy MACS0647−JD

Abstract JWST spectroscopy has revolutionized our understanding of galaxies in the early Universe. Covering wavelengths up to 5.3 μ m, NIRSpec can detect rest-frame optical H α emission lines out to z = 7 and O iii to z = 9.5. Observing these lines in more distant galaxies requires longer wavelength spectroscopy with MIRI. Here we present MIRI Medium Resolution Spectrograph integral field unit observations of the lensed galaxy merger MACS0647–JD at z = 10.165. With exposure times of 4.2 hr in each of two bands (SHORT and LONG), we detect H α at 9 σ , O iii λ 5008 at 11 σ , and O iii λ 4960 at 3 σ . Combined with previously reported NIRSpec spectroscopy that yielded seven emission lines including the auroral line O iii λ 4363, we present the first direct metallicity measurement of a z > 10 galaxy: 12 + log ( O / H ) = 7.79 ± 0.09 , or 0.13 − 0.03 + 0.02 Z ⊙ . This is similar to galaxies at z ∼ 4–9 with direct metallicity measurements, though higher than expected given the high specific star formation rate log(sSFR/yr −1 ) = −7.4 ± 0.3. We further constrain the ionization parameter log ( U ) = −1.9 ± 0.1, ionizing photon production efficiency log( ξ ion ) = 25.3 ± 0.1, and SFR = 5.0 ± 0.6 M ⊙ yr −1 within the past 10 Myr. These observations demonstrate the combined power of JWST NIRSpec and MIRI for studying galaxies in the first 500 million years.