Unveiling galaxy chemical enrichment mechanisms out to z~8 from direct determination of O & Ar abundances from JWST/NIRSPEC spectroscopy

Abstract

Galaxy chemical enrichment mechanisms have primarily been constrained by [$\alpha$/Fe] and [Fe/H] measurements of individual stars and integrated light from stellar populations. However such measurements are limited at higher redshifts (z>1). Recently, we proposed an analogous diagram of the oxygen-to-argon abundance ratio, log(O/Ar), vs Ar abundance, 12+log(Ar/H), as a new diagnostic window for emission nebulae. In this Letter, using robust line flux measurements including temperature sensitive auroral lines, we present direct determination of O and Ar abundances in nine SFGs from JWST/NIRSPEC spectra at z$\sim$1.3-7.7, and two more with Keck/MOSFIRE spectra at z$\sim$2.2. Utilising their positions on the log(O/Ar) vs 12+log(Ar/H) plane, we present the first inference of galaxy chemical enrichment mechanisms from an ensemble of galaxies. The SFGs at z$\sim$1.3-3.4 are consistent with the solar neighbourhood galactic chemical enrichment models of the Milky Way Galaxy that are driven by core-collapse and Type Ia supernovae. Such enrichment mechanisms thus occur at least out to z$\sim$3.4. However, the highest-redshift SFGs (z$\sim$3.6-7.7) have very low log(O/Ar) values, revealing a different enrichment process at z>3.6. Such low log(O/Ar) values may be caused by a rapid but intermittent star-formation and/or additional sources. The new diagnostic window for SFGs enables us to reveal the unique fingerprints of galaxy chemical enrichment out to cosmic dawn.