2024ApJ...969..148C

We provide one of the most comprehensive metallicity studies at z ∼ 4 by analyzing the UV/optical Hubble Space Telescope photometry and rest-frame Very Large Telescope (VLT)/FORS2 UV and VLT/XSHOOTER optical spectra of J0332-3557, a gravitationally lensed galaxy magnified by a factor of 20. With a 5σ detection of the auroral O III] λ1666 line, we are able to derive a direct gas metallicity estimate for our target. We find Z_gas$=12+\mathrm{log}({\rm{O}}/{\rm{H}})=8.26\pm 0.06$, which is compatible with an increase of both the gas fraction and the outflow metal loading factor from z ∼ 0 to z ∼ 4. J0332-3557 is the most metal-rich individual galaxy at z ∼ 4 for which the C/O ratio has been measured. We derive a low log(C/O) = -1.02 ± 0.2, which suggests that J0332-3557 is in the early stages of interstellar medium carbon enrichment driven mostly by massive stars. The low C/O abundance also indicates that J0332-3557 is characterized by a low star formation efficiency, higher yields of oxygen, and longer burst duration. We find that EW_C III]1906,9_ is as low as ∼3 Å, and the main drivers of the low EW_C III]1906,9_ are the higher gas metallicity and the low C/O abundance. J0332-3557 is characterized by one diffuse and two more compact regions ∼1 kpc in size. We find that the carbon emission mostly originates in the compact knots. Our study on J0332-3557 serves as an anchor for studies investigating the evolution of metallicity and C/O abundance across different redshifts.