A Comprehensive Metallicity Analysis of J0332−3557: Establishing a z ∼ 4 Anchor for Direct Gas Metallicity and C/O Abundance Investigations

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 + log ( O / H ) = 8.26 ± 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.