"Rethinking metallicity: the quest to measure galaxy abundances in the early universe"
Recent large near-infrared spectroscopic surveys have confirmed that star-forming galaxies at cosmic noon (z~2-3) exhibit nebular spectra that are distinct from their local counterparts. These differences reflect important changes in the characteristic physical conditions and chemical enrichment patterns of galaxies at high redshift, likely correlated with differences in their star formation histories relative to most z~0 galaxies. I will present new results from the Keck Baryonic Structure Survey (KBSS) that support this hypothesis and illustrate how lessons learned from spectroscopic surveys of z~2-3 galaxies inform our interpretation of galaxies’ nebular spectra across redshift. I have used photoionization models designed to reconcile the joint rest-UV-optical spectra of high-z star-forming galaxies to self-consistently infer the chemistry (O/H, Fe/H, and N/O) and nebular ionization and excitation conditions (including ionization parameter, U) in a sample of ~150 individual z~2-3 galaxies. Based on these results, which show that the majority of z~2-3 KBSS galaxies have moderate oxygen enrichment but sub-solar iron enrichment, I will argue that we should be studying galaxies' abundance patterns rather than continuing to rely on a single "metallicity" measurement to describe their chemical enrichment. Finally, I will discuss implications for measuring the intrinsic correlations between physical conditions across the z~2-3 galaxy sample (including the mass-metallicity relation), which are important for constraining theories of galaxy formation and evolution.