JWST reveals a possible z ∼ 11 galaxy merger in triply-lensed MACS0647–JD

Abstract Galaxies have formed from the repeated mergers of small star-forming clumps over cosmic time, with some small galaxies left over even today, such as the Magellanic clouds. JWST has now discovered two such small galaxies within the first 430 billion years, that are seen close to the very start of this process. MACS0647--JD is a triply-lensed z 11 galaxy originally discovered with the Hubble Space Telescope. Here we report new JWST imaging, which clearly resolves MACS0647--JD as having two components that are either merging galaxies or stellar complexes within a single galaxy. The brighter larger component ``A'' is intrinsically very blue (-2. 6), likely due to very recent star formation and no dust, and is spatially extended with an effective radius 70\, pc. The smaller component ``B'' appears redder (-2), likely because it is older (100--200\, Myr) with mild dust extinction (AV 0. 1\, mag), and a smaller radius 20\, pc. We identify galaxies with similar colors in a high-redshift simulation, finding their star formation histories to be out of phase. JWST NIRSpec observations planned for January 2023 will deliver a spectroscopic redshift and a more detailed study of the physical properties of MACS0647--JD.