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$\beta$ 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 $z\sim 2$ 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]/H$\beta$ and [N II]/H$\alpha$ values at high redshift arises from sample selection: when we consider only the most massive galaxies $M_{\ast }\sim 10^{10-11}{M}_{\odot }$, 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.

arXiv comment25 pages, 15 figures. Accepted for publication in ApJ

References in corpus (129)

• Planck 2015 results. XIII. Cosmological parameters
• Modules for Experiments in Stellar Astrophysics (MESA)
• The Host Galaxies of AGN
• The Origin of the Mass--Metallicity Relation: Insights from 53,000 Star-Forming Galaxies in the SDSS
• Modules for Experiments in Stellar Astrophysics (MESA): Giant Planets, Oscillations, Rotation, and Massive Stars
• The physical properties of star forming galaxies in the low redshift universe
• Modules for Experiments in Stellar Astrophysics (MESA): Binaries, Pulsations, and Explosions
• MESA Isochrones and Stellar Tracks (MIST). I: Solar-Scaled Models
• Theoretical Modeling of Starburst Galaxies
• Evolutionary tracks and isochrones for low- and intermediate-mass stars: from 0.15 to 7 M_sun, and from Z=0.0004 to 0.03
• MESA Isochrones and Stellar Tracks (MIST) 0: Methods for the construction of stellar isochrones
• The propagation of uncertainties in stellar population synthesis modeling I: The relevance of uncertain aspects of stellar evolution and the IMF to the derived physical properties of galaxies
• A Survey of Stellar Families: Multiplicity of Solar-Type Stars
• [O III]/[N II] as an Abundance Indicator at High Redshift
• Grids of stellar models with rotation - I. Models from 0.8 to 120 Msun at solar metallicity (Z = 0.014)
• MILES: A Medium resolution INT Library of Empirical Spectra
• Evolution of asymptotic giant branch stars II. Optical to far-infrared isochrones with improved TP-AGB models
• The Eighth Data Release of the Sloan Digital Sky Survey: First Data from SDSS-III
• Stellar Multiplicity
• The propagation of uncertainties in stellar population synthesis modeling III: model calibration, comparison, and evaluation
• Using Strong Lines to Estimate Abundances in Extragalactic HII Regions and Starburst Galaxies
• GIZMO: A New Class of Accurate, Mesh-Free Hydrodynamic Simulation Methods
• Simba: Cosmological Simulations with Black Hole Growth and Feedback
• AMAZE. I. The evolution of the mass-metallicity relation at z>3
• Binary Population and Spectral Synthesis Version 2.1: construction, observational verification and new results
• Rotating Massive Main-Sequence Stars I: Grids of Evolutionary Models and Isochrones
• The evolution of rotating stars
• An updated MILES stellar library and stellar population models
• The Stellar Population Histories of Early-Type Galaxies. II. Controlling Parameters of the Stellar Populations
• De Re Metallica: The cosmic chemical evolution of galaxies
• The chemical composition of metal-poor emission-line galaxies in the Data Release 3 of the Sloan Digital Sky Survey
• Strong Nebular Line Ratios in the Spectra of z~2-3 Star-forming Galaxies: First Results from KBSS-MOSFIRE
• Southern Massive Stars at High Angular Resolution: Observational Campaign and Companion Detection
• A systematic variation of the stellar initial mass function in early-type galaxies
• Reevaluating Old Stellar Populations
• The Mass-Metallicity Relation with the Direct Method on Stacked Spectra of SDSS Galaxies
• Database of Geneva stellar evolution tracks and isochrones for UBVRIJHKLL'M, HST-WFPC2, Geneva and Washington photometric systems
• The High Angular Resolution Multiplicity of Massive Stars
• The VLT-FLAMES Tarantula Survey VIII. Multiplicity properties of the O-type star population
• Understanding Galaxy Evolution through Emission Lines
• Nebular Emission from Star-Forming Galaxies
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• Theoretical Evolution of Optical Strong Lines across Cosmic Time
• Nebular Continuum and Line Emission in Stellar Population Synthesis Models
• Reconciling the Stellar and Nebular Spectra of High Redshift Galaxies
• A spectroscopic survey on the multiplicity of high-mass stars
• Grackle: a Chemistry and Cooling Library for Astrophysics
• Modelling the nebular emission from primeval to present-day star-forming galaxies
• MUFASA: Galaxy Formation Simulations With Meshless Hydrodynamics
• Environment and self-regulation in galaxy formation
• The MOSDEF Survey: Electron Density and Ionization Parameter at $z\sim 2.3$
• New calibrations for abundance determinations in HII regions
• Physical Conditions in a Young, Unreddened, Low Metallicity Galaxy at High Redshift
• New fully empirical calibrations of strong-line metallicity indicators in star forming galaxies
• Chemical abundances in high-redshift galaxies: A powerful new emission line diagnostic
• Early-Type Galaxy Archeology: Ages, Abundance Ratios, and Effective Temperatures from Full-Spectrum Fitting
• Nebular Emission Line Ratios in z~2-3 Star-Forming Galaxies with KBSS-MOSFIRE: Exploring the Impact of Ionization, Excitation, and Nitrogen-to-Oxygen Ratio
• Counting Low-Mass Stars in Integrated Light
• Gravitational torque-driven black hole growth and feedback in cosmological simulations
• New insights into the stellar content and physical conditions of star-forming galaxies at z = 2-3 from spectral modelling
• Evidence of Cosmic Evolution of the Stellar Initial Mass Function
• SDSS-IV MaNGA: The Impact of Diffuse Ionized Gas on Emission-line Ratios, Interpretation of Diagnostic Diagrams, and Gas Metallicity Measurements
• The MOSDEF Survey: Excitation Properties of $z\sim 2.3$ Star-forming Galaxies
• HYPERION: An open-source parallelized three-dimensional dust continuum radiative transfer code
• Ultraviolet Emission Lines in Young Low Mass Galaxies at z~2: Physical Properties and Implications for Studies at z>7
• From binaries to multiples I: Data on F and G dwarfs within 67 pc of the Sun
• Physical Properties of Emission-Line Galaxies at z ~ 2 from Near-Infrared Spectroscopy with Magellan FIRE
• Metallicities and Physical Conditions in Star-forming Galaxies at z~1.0-1.5
• The MOSDEF Survey: Direct-Method Metallicities and ISM Conditions at $z\sim 1.5-3.5$
• CHAOS I: Direct Chemical Abundances for HII Regions in NGC 628
• The Dust-to-Gas and Dust-to-Metals Ratio in Galaxies from z=0-6
• Rest-Frame Optical Spectra of Three Strongly Lensed Galaxies at z~2
• Chemical Abundances of DEEP2 Star-forming Galaxies at 1.0<z<1.5
• CHAOS III: Gas-Phase Abundances in NGC5457
• The oxygen abundance in the inner HII regions of M101. Implications for the calibration of strong-line metallicity indicators
• ISM excitation and metallicity of star-forming galaxies at z~3.3 from near-IR spectroscopy
• The FMOS-COSMOS survey of star-forming galaxies at z~1.6. IV: Excitation state and chemical enrichment of the interstellar medium
• A Window On The Earliest Star Formation: Extreme Photoionization Conditions of a High-Ionization, Low-Metallicity Lensed Galaxy at z~2
• Ionized gaseous nebulae chemical abundance determination using the direct method
• Measuring the Physical Conditions in High-Redshift Star-Forming Galaxies: Insights from KBSS-MOSFIRE
• Gravitationally Lensed Galaxies at 2<z<3.5: Direct Abundance Measurements of Lya Emitters
• A Magnetically-Supported Photodissociation Region in M17
• First Spectroscopic Evidence for High Ionization State and Low Oxygen Abundance in Lya Emitters
• The Nature of Extreme Emission Line Galaxies at z=1-2: Kinematics and Metallicities from Near-Infrared Spectroscopy
• Chemical element ratios of SDSS early-type galaxies
• CHAOS II: Gas-Phase Abundances in NGC 5194
• The Physical Conditions, Metallicity and Metal Abundance Ratios In a Highly Magnified Galaxy at z = 3.6252
• The Low Mass End of the Fundamental Relation for Gravitationally Lensed Star Forming Galaxies at 1<z<6
• The Ages, Metallicities and Alpha Element Enhancements of Globular Clusters in the Elliptical NGC 5128: A Homogeneous Spectroscopic Study with Gemini/GMOS
• Synthetic nebular emission from massive galaxies I: origin of the cosmic evolution of optical emission-line ratios
• Probing Cosmic Dawn with Emission Lines: Predicting Infrared and Nebular Line Emission for ALMA and JWST
• Biases in metallicity measurements from global galaxy spectra: the effects of flux-weighting and diffuse ionized gas contamination
• Stars were born in significantly denser regions in the early Universe
• The MOSDEF-LRIS Survey: The Interplay Between Massive Stars and Ionized Gas in High-Redshift Star-Forming Galaxies
• Temperature-based metallicity measurements at z=0.8: direct calibration of strong-line diagnostics at intermediate redshift
• Testing metallicity indicators at z~1.4 with the gravitationally lensed galaxy CASSOWARY 20
• 3D-HST Grism Spectroscopy of a Gravitationally Lensed, Low-metallicity Starburst Galaxy at z=1.847
• LBT/LUCIFER Observations of the z~2 Lensed Galaxy J0900+2234
• Reproducing sub-millimetre galaxy number counts with cosmological hydrodynamic simulations
• The MOSDEF Survey: Detection of [OIII]$\lambda$4363 and the direct-method oxygen abundance of a star-forming galaxy at z=3.08
• The Physical Conditions of a Lensed Star-forming Galaxy at z=1.7
• Evolution of N/O Abundance Ratios and Ionization Parameters from z~0 to 2 Investigated by the Direct Temperature Method
• The binarity of Milky Way F,G,K stars as a function of effective temperature and metallicity
• Powderday: Dust Radiative Transfer for Galaxy Simulations
• The MOSDEF-LRIS Survey: The Connection Between Massive Stars and Ionized Gas in Individual Galaxies at $z\sim 2$
• The MOSDEF Survey: Sulfur Emission-line Ratios Provide New Insights into Evolving ISM Conditions at High Redshift
• Abundance determination from global emission-line SDSS spectra: exploring objects with high N/O ratios
• Nebular and stellar properties of a metal poor HII galaxy at z=3.36
• Diffuse ionized gas and its effects on nebular metallicity estimates of star-forming galaxies
• First direct metallicity measurement of a lensed star-forming galaxy at z=1.7
• The ionized gas in the central region of NGC 5253: 2D mapping of the physical and chemical properties
• Stellar Populations of Highly Magnified Lensed Galaxies: Young Starbursts at z~2
• Piecing together the puzzle of NGC 5253: abundances, kinematics and WR stars
• The MOSDEF Survey: A Comprehensive Analysis of the Rest-optical Emission-line Properties of $z\sim 2.3$ Star-forming Galaxies
• Star Cluster Mass and Age Distributions of Two Fields in M83 Based on HST/WFC3 Observations
• Star-forming Clumps in Local Luminous Infrared Galaxies
• Spectroscopic Observation and Analysis of HII regions in M33 with MMT: Temperatures and Oxygen Abundances
• The Photometric Properties of Galaxies in the Early Universe
• Luminosity Dependence and Redshift Evolution of Strong Emission-line Diagnostics in Star-Forming Galaxies
• The [α/Fe]-[Fe/H] relation in the E-MOSAICS simulations: its connection to the birth place of globular clusters and the fraction of globular cluster field stars in the bulge
• Massive Star Cluster Formation and Destruction in Luminous Infrared Galaxies in GOALS
• Star-forming galaxies are predicted to lie on a fundamental plane of mass, star formation rate and α-enhancement
• The Detection of [O III]$\lambda$4363 in a Lensed, Dwarf Galaxy at $z$ = 2.59: Testing Metallicity Indicators and Scaling Relations at High Redshift and Low Mass
• Physical conditions of the interstellar medium in star-forming galaxies at z~1.5
• Testing strong line metallicity diagnostics at z~2
• On the origin of nitrogen at low metallicity
• On the relationship between auroral and nebular oxygen line intensities in spectra of HII regions
• What Drives the Redshift Evolution of Strong Emission Line Ratios?
• FirstLight IV: Diversity in sub-L${}_{\ast }$ galaxies at cosmic dawn

Cited by in corpus (12)

• Quenching and the UVJ diagram in the SIMBA cosmological simulation
• The resolved chemical abundance properties within the interstellar medium of star-forming galaxies at $\text{z}\approx 1.5$
• An X-ray Census of Fast Radio Burst Host Galaxies: Constraints on AGN and X-ray Counterparts
• First Light And Reionisation Epoch Simulations (FLARES) XI: [OIII] emitting galaxies at $5<z<10$
• First Light And Reionisation Epoch Simulations (FLARES) XII: The consequences of star-dust geometry on galaxies in the EoR
• Euclid preparation. Optical emission-line predictions of intermediate-z galaxy populations in GAEA for the Euclid Deep and Wide Surveys
• TODDLERS: A New UV to Millimeter Emission Library for Star-Forming Regions. II. Star Formation Rate Indicators Using Auriga Zoom Simulations
• MAMMOTH-MOSFIRE: Environmental Effects on Galaxy Interstellar Medium at $z\sim 2$
• Cloudy-Maraston: Integrating nebular continuum and line emission with the Maraston stellar population synthesis models
• CECILIA: Ultra-Deep Rest-Optical Spectra of Faint Galaxies at Cosmic Noon
• Recovering the properties of the interstellar medium through integrated spectroscopy: application to the z~0 ECO volume-limited star-forming galaxy sample
• Euclid: A blue galaxy population and a brightest cluster galaxy in the making in a $z\sim 1.74$ MaDCoWS2 galaxy cluster candidate