Early Results from GLASS–JWST. XXI. Rapid Asembly of a Galaxy at z = 6.23 Revealed by Its C/O Abundance

The abundance of carbon relative to oxygen (C/O) is a promising probe of star formation history in the early universe, as the ratio changes with time due to production of these elements by different nucleosynthesis pathways. We present a measurement of log(C/O) = -1.01 +/- 0.12 (stat) +/- 0.15 (sys) in a z = 6.23 galaxy observed as part of the GLASS-JWST Early Release Science Program. Notably, we achieve good precision thanks to the detection of the rest-frame ultraviolet O III], CIII], and C IV emission lines delivered by JWST/NIRSpec. The C/O abundance is similar to 0.8 dex lower than the solar value and is consistent with the expected yield from core-collapse supernovae, indicating that longer-lived intermediate-mass stars have not fully contributed to carbon enrichment. This in turn implies rapid buildup of a young stellar population with age less than or similar to 100 Myr in a galaxy seen similar to 900 Myr after the big bang. Our chemical abundance analysis is consistent with spectral energy distribution modeling of JWST/NIRCam photometric data, which indicates a current stellar mass log M-star/M-circle dot = 8.4(-0.2)(+0.4) and specific star formation rate similar or equal to 20 Gyr(-1). These results showcase the value of chemical abundances and C/O in particular to study the earliest stages of galaxy assembly.