TY - JOUR
T1 - The BPT Diagram in Cosmological Galaxy Formation Simulations: Understanding the Physics Driving Offsets at High-Redshift
AU - Garg, Prerak
AU - Narayanan, Desika
AU - Byler, Nell
AU - Sanders, Ryan L.
AU - Shapley, Alice E.
AU - Strom, Allison L.
AU - Davé, Romeel
AU - Hirschmann, Michaela
AU - Lovell, Christopher C.
AU - Otter, Justin
AU - Popping, Gergö
AU - Privon, George C.
N1 - 25 pages, 15 figures. Accepted for publication in ApJ
Funding Information:
The authors would like to thank Chuck Steidel for providing KBSS data for our observational comparisons. simba was run using the DiRAC@Durham facility managed by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility. The equipment was funded by BEIS capital funding via STFC capital grants ST/P002293/1, ST/R002371/1, and ST/S002502/1, Durham University, and STFC operations grant ST/R000832/1. R.D. acknowledges support from the Wolfson Research Merit Award program of the U.K. Royal Society. P.G. and D.N. were funded by NSF AST-1909153.
Publisher Copyright:
© 2022. The Author(s). Published by the American Astronomical Society.
PY - 2022/2/15
Y1 - 2022/2/15
N2 - 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.
AB - 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.
KW - astro-ph.GA
U2 - 10.3847/1538-4357/ac43b8
DO - 10.3847/1538-4357/ac43b8
M3 - Article
SN - 0004-637X
VL - 926
SP - 1
EP - 18
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1
M1 - 80
ER -