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Date: 4/26/2024

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

JWST MIRI coverage of the MACS0647 field by GO 4246 (PI: Abdurro’uf) overlaid on the composite color image of JWST and HST. Top left: MIRI MRS IFU spectroscopy on the brightest lensed image, MACS0647-JD1, including all three clumps A, B, and C. All three components are observed in the FoV in all 4 dithers. Bottom left: The second brightest lensed image, MACS0647-JD2, is included in the MIRI imaging field of view. Right: Full image of the simultaneous MIRI imaging and MRS coverage. Note a bright 8th magnitude star lurks just below and right off the edge of the HST field of view. Two zoom insets show the JWST NIRCam images of JD1 and JD2. Abstract: JWST spectroscopy has revolutionized our understanding of galaxies in the early universe. Covering wavelengths up to 5.3µm, NIRSpec can detect rest-frame optical emission lines Ha 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 Ha at 9s, [O III]?5008 at 11s, and [O III]?4960 at 3s. Combined with previously reported NIRSpec spectroscopy that yields seven emission lines including the auroral line [O III]?4363, we present the first direct metallicity measurement of a z>10 galaxy: 12+log(O/H)=7.79±0.09, or 0.13+0.02-0.03Z?. 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(?ion) = 25.3±0.1, and star formation rate 5.0±0.6M?/yr within the past 10Myr. These observations demonstrate the combined power of JWST NIRSpec and MIRI for studying galaxies in the first 500 million years.