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

TL;DR

This study uses cosmological simulations and photoionization modeling to understand why high-redshift galaxies appear offset in the BPT diagram, attributing the shift mainly to sample selection and physical conditions.

Contribution

It demonstrates that the observed BPT offset at high redshift results from sample selection effects and explores physical parameter variations that can reproduce the observed shifts.

Findings

High-mass galaxies at high redshift show BPT offsets due to high metallicities.

Decreasing ionization parameter or increasing N/O ratio can mimic high-redshift BPT shifts.

Deeper observations of lower-mass galaxies will align high-redshift and local galaxy distributions.

Abstract

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.