The Role of Star Formation and Morphology in the Mass-Metallicity Relation of Galaxies at Cosmic Noon

Abstract

We investigate the dependence of the mass-metallicity relation (MZR) on star formation and galaxy morphology at 1.7 1000 galaxies. The observations span >85 arcmin^2 from the pure-parallel PASSAGE and OutThere surveys. We derive gas-phase metallicities using a Bayesian framework that combines multiple strong emission-line ratios calibrated against direct auroral-line metallicity measurements from JWST. We constrain stellar masses, star formation rates (SFRs), and morphological properties using ancillary HST and JWST imaging. We stack spectra by stellar mass to constrain the MZR over log(M*/M)~6.5-11, and find that it is well described by a second-order polynomial, with a steep low-mass slope and metallicities<10% solar at log(M*/M)<7.5. Additional stacking by SFR and morphology reveals strong evidence for a fundamental metallicity relation (FMR). Specifically, at log(M*/M)<9.5, galaxies with lower SFRs are more metal-rich than those with higher SFRs at fixed mass. At log(M*/M)<9.5, we find that larger galaxies, and those having lower SFR surface densities, bulges, and/or spiral features are significantly metal rich relative to their smaller, higher SFR surface density, bulge-less, and/or non-spiral counterparts. These results suggest the secondary dependencies of the MZR reflect variations in the baryon cycle, such that recent metal-poor gas accretion drives concentrated star formation and metallicity dilution, while structurally developed galaxies retain more chemically enriched gas reservoirs.