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

Nikko J. Cleri, Grace M. Olivier, Bren E. Backhaus, Joel Leja, Casey Papovich, Jonathan R. Trump, Pablo Arrabal Haro, Véronique Buat, Denis Burgarella, Emilie Burnham, Antonello Calabrò, Jonathan H. Cohn, Justin W. Cole, Kelcey Davis, Mark Dickinson, Steven L. Finkelstein, Ray Garner, Michaela Hirschmann, Weida Hu, Taylor A. Hutchison, Dale D. Kocevski, Anton M. Koekemoer, Rebecca L. Larson, Zach J. Lewis, Michael v. Maseda, Lise-Marie Seillé, Raymond C. Simons
June, 2025
PDF Cite DOI arXiv
Figure 4 of Cleri et al. (2025).

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 (Zgas), 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 Zgas. The OHNO diagram is especially contaminated in the AGN region by stellar models with high log U and low Zgas, 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.

Type
2
Publication
In The Astrophysical Journal
Ray Garner
Ray Garner
TAMU Astronomy Postdoctoral Researcher

I’m a scientist, Star Wars fan, and amateur photographer raised in Georgia. My research interests include galaxy evolution, star formation, satellite galaxies, and nebular diagnostics.