The BPT Diagram in Cosmological Galaxy Formation Simulations:
  Understanding the Physics Driving Offsets at High-Redshift
  The Baldwin, Philips, & Terlevich diagram of [O III]/H$\beta$ vs. [N
II]/H$\alpha$ (hereafter N2-BPT) has long been used as a tool for classifying
galaxies based on the dominant source of ionizing radiation. Recent
observations have demonstrated that galaxies at $z\sim2$ reside offset from
local galaxies in the N2-BPT space. In this paper, we conduct a series of
controlled numerical experiments to understand the potential physical processes
driving this offset. We model nebular line emission in a large sample of
galaxies, taken from the SIMBA cosmological hydrodynamic galaxy formation
simulation, using the CLOUDY photoionization code to compute the nebular line
luminosities 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 from
sample selection: when we consider only the most massive galaxies $M_* \sim
10^{10-11} M_\odot$, the offset naturally appears, due to their high
metallicities. We predict that deeper observations that probe lower-mass
galaxies will reveal galaxies that lie on a locus comparable to $z\sim 0$
observations. Even when accounting for sample selection effects, we find that
there is a subtle mismatch between simulations and observations. To resolve
this discrepancy, we investigate the impact of varying ionization parameters, H
II region densities, gas-phase abundance patterns, and increasing radiation
field hardness on N2-BPT diagrams. We find that either decreasing the
ionization parameter or increasing the N/O ratio of galaxies at fixed O/H can
move galaxies along a self-similar arc in N2-BPT space that is occupied by
high-redshift galaxies.
