Description
The fundamental metallicity relation (FMR) — the three-way trend between galaxy stellar masses, star-formation rates (SFRs) and gaseous metallicities — remains amongst the most studied extragalactic relations. Furthermore, more compact galaxies possess higher average metallicities at a given stellar mass, such that metallicity correlates particularly tightly with gravitational potential. Simulations support a shared origin for these trends relating to long-term variations in gas inflow histories; however, differences in simulated and observed galaxy samples make it unclear if this holds for real galaxies. Here, we revisit both results for star-forming galaxies, using MaNGA observations to probe galaxies’ metallicities at one effective radius. We confirm the FMR while also confirming equivalent relations for stellar metallicity (FMR) and gaseous N/O (fundamental nitrogen relation, FNR). We find that all three of these relations persist if we consider gravitational potential in place of stellar mass and/or if we consider stellar ages in place of SFR, with the gaseous relations strengthening significantly when potential is considered. The gaseous FMR disappears at high masses/potentials, while the FNR persists and the FMR strengthens significantly.
From our results, we present a unified interpretation of galaxies’ O/H, N/O, and stellar metallicities. Higher potentials are associated with higher equilibrium metallicities and earlier star-formation histories (SFHs), producing tight potential-abundance relations. SFR variations at low potential are associated with short-term SFH variations from gas inflows, mostly affecting gas abundances, while SFR variations at high potentials are instead associated with longer-term SFH variations.
