Optical Strong Line Ratios Cannot Distinguish between Stellar Populations and Accreting Black Holes at High Ionization Parameters and Low Metallicities

Abstract High-redshift observations from JWST indicate that optical strong line ratios do not carry the same constraining power as they do at low redshifts. Critically, this prevents a differentiation between stellar and black hole accretion-driven ionization, thereby obscuring both active galactic nuclei (AGN) demographics and star formation rates. To investigate this, we compute a large suite of photoionization models with Cloudy powered by stellar populations and accreting black holes over a large grid of ages, metallicities, initial mass functions, binarities, ionization parameters, densities, and black hole masses. We use these models to test three rest-frame optical diagnostics designed to separate ionizing sources at low redshifts: the [N ii ]-BPT, VO87, and OHNO diagrams. We show that these diagnostics are strongly driven by the ionization parameter (log U ) and the gas-phase metallicity ( Z gas ), often more so than the ionizing spectrum itself; there is significant overlap between stellar population and accreting black hole models at high log U and low Z gas . The OHNO diagram is especially contaminated in the AGN region by stellar models with high log U and low Z gas , consistent with high-redshift observations. We show that the [N ii ]-BPT, VO87, or OHNO diagrams are most sensitive to the shape of the <54 eV ionizing continuum, an energy regime in which stellar populations and black hole accretion disk models can be highly degenerate. Finally, we discuss the potential for emission lines that trace the >54 eV ionizing continuum to differentiate between ionizing sources more effectively than the [N ii ]-BPT, VO87, or OHNO diagrams alone.

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Publication Details

Journal
The Astrophysical Journal
Published
2025-11-21
DOI
https://doi.org/10.3847/1538-4357/ae0f17
Citations
1
Primary Topic
Astrophysical Phenomena and Observations
Type
article
Field-Weighted Citation Impact
1.15
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article

Optical Strong Line Ratios Cannot Distinguish between Stellar Populations and Accreting Black Holes at High Ionization Parameters and Low Metallicities

Bren E. Backhaus, V. Buat, Ray Garner, D. Burgarella et al.
1 citations
The Astrophysical Journal
Astrophysical Phenomena and Observations
1.15
article

Optical Strong Line Ratios Cannot Distinguish between Stellar Populations and Accreting Black Holes at High Ionization Parameters and Low Metallicities

Bren E. Backhaus, V. Buat, Ray Garner, D. Burgarella, Rebecca L. Larson, Justin W. Cole, Pablo Arrabal Haro, Casey Papovich, Anton M. Koekemoer, Antonello Calabrò, Raymond C. Simons, Zach J. Lewis, Kelcey Davis, Jonathan H. Cohn, Joel Leja, Grace M. Olivier, Nikko J. Cleri, Jonathan R. Trump, Emilie Burnham, Steven L. Finkelstein, Lise-Marie Seillé, Mark Dickinson, Dale D. Kocevski, Michael V. Maseda, Weida Hu, Taylor A. Hutchison, Michaela Hirschmann
article en
1 citations

Abstract

Abstract High-redshift observations from JWST indicate that optical strong line ratios do not carry the same constraining power as they do at low redshifts. Critically, this prevents a differentiation between stellar and black hole accretion-driven ionization, thereby obscuring both active galactic nuclei (AGN) demographics and star formation rates. To investigate this, we compute a large suite of photoionization models with Cloudy powered by stellar populations and accreting black holes over a large grid of ages, metallicities, initial mass functions, binarities, ionization parameters, densities, and black hole masses. We use these models to test three rest-frame optical diagnostics designed to separate ionizing sources at low redshifts: the [N ii ]-BPT, VO87, and OHNO diagrams. We show that these diagnostics are strongly driven by the ionization parameter (log U ) and the gas-phase metallicity ( Z gas ), often more so than the ionizing spectrum itself; there is significant overlap between stellar population and accreting black hole models at high log U and low Z gas . The OHNO diagram is especially contaminated in the AGN region by stellar models with high log U and low Z gas , consistent with high-redshift observations. We show that the [N ii ]-BPT, VO87, or OHNO diagrams are most sensitive to the shape of the <54 eV ionizing continuum, an energy regime in which stellar populations and black hole accretion disk models can be highly degenerate. Finally, we discuss the potential for emission lines that trace the >54 eV ionizing continuum to differentiate between ionizing sources more effectively than the [N ii ]-BPT, VO87, or OHNO diagrams alone.

The Astrophysical JournalVol. 994(2)
Dartmouth College (US), Centre National de la Recherche Scientifique (FR), Space Telescope Science Institute (US), Goddard Space Flight Center (US), Pennsylvania State University (US), University of Wisconsin–Madison (US), University of Connecticut (US), University of Kansas (US), Providence College (US), Aix-Marseille Université (FR), Colby College (US), Astronomical Observatory of Rome (IT), Mitchell Institute (US), Université Bourgogne Franche-Comté (FR), NSF NOIRLab (US), Deleted Institution, École Polytechnique Fédérale de Lausanne (CH), The University of Texas at Austin (US), Texas A&M University (US)
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Astrophysical Phenomena and Observations
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