The Puzzling Properties of the MACS1149-JD1 Galaxy at z = 9.11

We analyze new JWST NIRCam and NIRSpec data on the redshift 9.11 galaxy MACS1149-JD1 (hereafter JD1). Our NIRCam imaging data reveal that JD1 comprises three spatially distinct components. Our spectroscopic data indicate that JD1 appears dust free but is already enriched, $12+\mathrm{log}({\rm{O}}/{\rm{H}})={7.90}_{-0.05}^{+0.04}$ . We also find that the carbon and neon abundances in JD1 are below the solar abundance ratio. Particularly the carbon under-abundance is suggestive of recent star formation where Type II supernovae have already enriched the interstellar medium (ISM) in oxygen but intermediate mass stars have not yet enriched the ISM in carbon. A recent burst of star formation is also revealed by the star formation history derived from NIRCam photometry. Our data do not reveal the presence of a significant amount of old populations, resulting in a factor of ~7× smaller stellar mass than previous estimates. Thus, our data support the view that JD1 is a young galaxy.