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

Prerak Garg*, Desika Narayanan, Nell Byler, Ryan L. Sanders, Alice E. Shapley, Allison L. Strom, Romeel Davé, Michaela Hirschmann, Christopher C. Lovell, Justin Otter, Gergö Popping, George C. Privon

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

The Baldwin, Philips, & Terlevich diagram of [O iii]/Hβ versus [N ii]/Hα (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 ∼ 2 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]/Hβ and [N ii]/Hα values at high redshift arises from sample selection: when we consider only the most massive galaxies M* ∼ 1010–11 M, 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 ∼ 0 observations. Even when accounting for samples-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.
Original languageEnglish
Article number80
Pages (from-to)1-18
Number of pages18
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 15 Feb 2022

Keywords / Materials (for Non-textual outputs)

  • astro-ph.GA


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