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

Abstract

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.