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The Baldwin, Philips, & Terlevich diagram of [O III]/H$\beta$ vs. [NII]/H$\alpha$ (hereafter N2-BPT) has long been used as a tool for classifyinggalaxies based on the dominant source of ionizing radiation. Recentobservations have demonstrated that galaxies at $z\sim2$ reside offset fromlocal galaxies in the N2-BPT space. In this paper, we conduct a series ofcontrolled numerical experiments to understand the potential physical processesdriving this offset. We model nebular line emission in a large sample ofgalaxies, taken from the SIMBA cosmological hydrodynamic galaxy formationsimulation, using the CLOUDY photoionization code to compute the nebular lineluminosities from H II regions. We find that the observed shift toward higher[O III]/H$\beta$ and [N II]/H$\alpha$ values at high redshift arises fromsample selection: when we consider only the most massive galaxies $M_* \sim10^{10-11} M_\odot$, the offset naturally appears, due to their highmetallicities. We predict that deeper observations that probe lower-massgalaxies will reveal galaxies that lie on a locus comparable to $z\sim 0$observations. Even when accounting for sample selection effects, we find thatthere is a subtle mismatch between simulations and observations. To resolvethis discrepancy, we investigate the impact of varying ionization parameters, HII region densities, gas-phase abundance patterns, and increasing radiationfield hardness on N2-BPT diagrams. We find that either decreasing theionization parameter or increasing the N/O ratio of galaxies at fixed O/H canmove galaxies along a self-similar arc in N2-BPT space that is occupied byhigh-redshift galaxies.

The BPT Diagram in Cosmological Galaxy Formation Simulations: Understanding the Physics Driving Offsets at High-Redshift