JWST reveals a possible $z\sim 11$ galaxy merger in triply-lensed MACS0647$-$JD

Abstract

MACS0647$-$JD is a triply-lensed $z\sim 11$ 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 $\sim 10^8{M}_{\odot }$ and radii $r<100\mathrm{pc}$. The brighter larger component "A" is intrinsically very blue (${\hat{I}}^2\sim -2.6$), likely due to very recent star formation and no dust, and is spatially extended with an effective radius $\sim 70\mathrm{pc}$. The smaller component "B" appears redder (${\hat{I}}^2\sim -2$), likely because it is older ($100-200\mathrm{Myr}$) with mild dust extinction ($A_V\sim 0.1\mathrm{mag}$), and a smaller radius $\sim 20\mathrm{pc}$. 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 $\sim 400\mathrm{pc}$, we may be witnessing a galaxy merger 400 million years after the Big Bang. We also identify a candidate companion galaxy C $\sim 3\mathrm{kpc}$ away, likely destined to merge with galaxies A and B. The combined light from galaxies A+B is magnified by factors of $\sim$8, 5, and 2 in three lensed images JD1, 2, and 3 with F356W fluxes $\sim 322$, $203$, $86\mathrm{nJy}$ (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 $z=10.6\pm 0.3$ based on photometry measured in 6 NIRCam filters spanning $1-5\hat{\mathrm{I}}¼\mathrm{m}$, out to $4300\tilde{A}$ 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.

27 pages, 14 figures, accepted for publication in ApJL