2025ApJ...993...70H

Investigating the metal enrichment in the early Universe helps us constrain theories about the first stars and study the ages of galaxies. The lensed galaxy MACS0647-JD at z = 10.17 is the brightest galaxy known at z > 10. Previous work analyzing JWST NIRSpec and MIRI data yielded a direct metallicity $12+{\rm{log}}({\rm{O}}/{\rm{H}})\,=7.79\pm 0.09$ (∼0.13 Z_⊙) and electron density ${\rm{log}}({n}_{e}/{{\rm{cm}}}^{-3})=2.9\pm 0.5$, the most distant such measurements to date. Here we estimate the direct C/O abundance for the first time at z > 10, finding a subsolar ${\rm{log}}({\rm{C}}/{\rm{O}})\,=-0.4{4}_{-0.07}^{+0.06}$. This is higher than other z > 6 galaxies with direct C/O measurements, likely due to higher metallicity. It is also slightly higher than galaxies in the local Universe with similar metallicity. This may suggest a very efficient and rapid burst of star formation, a low effective oxygen abundance yield, or the presence of unusual stellar populations, including supermassive stars. Alternatively, the strong C III] λλ1907,1909 emission (14 ± 3 Å rest-frame equivalent width may originate from just one of the two component star clusters JDB (r ∼ 20 pc). Future NIRSpec integral field unit spectroscopic observations of MACS0647-JD will be promising for disentangling C/O in the two components to constrain the chemistry of individual star clusters just 460 Myr after the Big Bang.