James Webb Space Telescope Feed Post

JWST reveals a possible z \sim 11 galaxy merger in triply-lensed MACS0647-JD Published: 10/25/2022 7:18:50 PM Updated: 6/1/2023 1:12:31 AM

Paper abstract: 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. Both are very small, with stellar masses ~10^8\,M_\odot and radii r<100\,\rm pc. The brighter larger component "A" is intrinsically very blue (\beta~-2.6), likely due to very recent star formation and no dust, and is spatially extended with an effective radius ~70\,\rm pc. The smaller component "B" appears redder (\beta~-2), likely because it is older (100-200\,\rm Myr) with mild dust extinction (A_V~0.1\,\rm mag), and a smaller radius ~20\,\rm pc. We identify galaxies with similar colors in a high-redshift simulation, finding their star formation histories to be out of phase. With an estimated stellar mass ratio of roughly 2:1 and physical projected separation ~400\,\rm pc, we may be witnessing a galaxy merger 400 million years after the Big Bang. We also identify a candidate companion galaxy C ~3\,{\rm kpc} away, likely destined to merge with galaxies A and B. The combined light from galaxies A+B is magnified by factors of ~8, 5, and 2 in three lensed images JD1, 2, and 3 with F356W fluxes ~322, 203, 86\,\rm nJy (AB mag 25.1, 25.6, 26.6). MACS0647-JD is significantly brighter than other galaxies recently discovered at similar redshifts with JWST. Without magnification, it would have AB mag 27.3 (M_{UV}=-20.4). With a high confidence level, we obtain a photometric redshift of z=10.6\pm0.3 based on photometry measured in 6 NIRCam filters spanning 1-5\rm\mu m, out to 4300\,\r{A} rest-frame. JWST NIRSpec observations planned for January 2023 will deliver a spectroscopic redshift and a more detailed study of the physical properties of MACS0647-JD.