JWST MIRI detections of Hα and [O III] and direct metallicity measurement of the z=10.17 lensed galaxy MACS0647-JD

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 emission lines Hα 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 MRS IFU observations of the lensed galaxy merger MACS0647-JD at z = 10.165. With exposure times of 4.2 hours in each of two bands, we detect Hα at 9σ, [O III]λ5008 at 11σ, and [O III]λ4960 at 3σ. Combined with previously reported NIRSpec spectroscopy that yields seven emission lines including the auroral line [O III]λ4363, we present the first direct metallicity measurement of a z > 10 galaxy: 12+ log(O/H)= 7.79±0.09, or 0.13^+0.02_-0.03 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 star formation rate 5.0±0.6 M_⊙/ yr 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.