Description
Galactic chemical enrichment mechanisms have primarily been constrained by alpha-enrichment ([α/Fe]) and metallicity ([Fe/H]) measurements from deep absorption-line spectra of individual stars in the Milky Way (MW) and some local group dwarf galaxies. At larger distances out to high-redshifts (z>2), such measurements are only possible from integrated light from galaxies, almost exclusively from massive early-types. For emission nebulae (originally through analysing the direct chemical abundances of Planetary Nebulae and HII regions in the Andromeda galaxy), we found that the oxygen-to-argon abundance ratio, log(O/Ar), vs Ar abundance, 12+log(Ar/H), is analogous to [α/Fe] vs [Fe/H] for stars. This unique diagnostic plane allows us to probe chemical enrichment from the integrated emission-line spectra of star-forming galaxies (SFGs), which are the vast majority of galaxies in the universe, with their fraction increasing with increasing redshift. Utilising this diagnostic window, at low redshifts (z<0.3) with Sloan-Digital Sky Survey (SDSS) observations of ~800 SFGs, we observationally show that galaxy chemical enrichment history is driven primarily by the interplay of core-collapse and Type Ia supernovae, and how the impact of prevalent chemical enrichment mechanisms varies with galaxy mass. With a smaller sample of 11 SFGs at higher redshifts (z~1.3-7.7) with JWST/NIRSPEC and Keck/MOSFIRE, we show that MW-like chemical enrichment processes occur at least out to z~4, beyond which rapid but intermittent star-formation may be at play. This new O & Ar abundance based diagnostic window for emission nebulae will enable us to reveal the unique fingerprints of galaxy chemical enrichment all the way out to cosmic dawn.
