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

Abstract The Baldwin, Philips, & Terlevich diagram of [O iii ]/H ? versus [N ii ? (hereafter N2-BPT) has long been used as a tool for classifying galaxies based on the dominant source ionizing radiation. Recent observations have demonstrated that at z ? 2 reside offset from local in N2-BPT space. In this paper, we conduct series controlled numerical experiments to understand potential physical processes driving offset. We model nebular line emission large sample galaxies, taken simba cosmological hydrodynamic galaxy formation simulation, using cloudy photoionization code compute luminosities H regions. find observed shift toward higher and values high redshift arises selection: when consider only most massive M * 10 10–11 ? , naturally appears, due their metallicities. predict deeper probe lower-mass will reveal lie locus comparable 0 observations. Even accounting samples-selection effects, there is subtle mismatch between simulations To resolve discrepancy, investigate impact varying ionization parameters, region densities, gas-phase abundance patterns, increasing radiation field hardness diagrams. either decreasing parameter or N/O ratio fixed O/H can move along self-similar arc space occupied by high-redshift galaxies.