The redshift evolution of strong emission line ratios reflects a link between N/O ratio and galaxy stellar mass
Daniel Masters

The offset of high redshift star-forming galaxies in the classical [OIII]/H-beta vs. [NII]/H-alpha BPT diagnostic diagram in comparison with the local star-forming galaxy sequence is now well established. The physical origin of the shift is the subject of some debate, with potentially important implications for metallicity estimation at all redshifts. I will present results from an investigation of the BPT shift using a sample of ~100,000 star-forming galaxies from SDSS DR12. This sample (which includes numerous high-redshift "analogs") lets us determine how galaxy physical properties -- in particular, star formation rate density, ionization parameter, N/O ratio, and stellar mass -- drive position in key emission line diagnostic diagrams. I will present evidence that a relation between the nitrogen-to-oxygen (N/O) ratio and galaxy stellar mass underlies the observed BPT offset, and is more fundamental than the well-studied relation between N/O and O/H. The relation between N/O ratio and stellar mass induces a mass-dependence in the BPT diagram, such that the BPT shift observed in high redshift galaxies reflects the evolution of the mass-metallicity (MZ) relation. I will discuss implications of this result for metallicity measurements based on strong lines at high redshift, as well as for the proposed fundamental metallicity relation (FMR) between metallicity, star-formation rate, and stellar mass.