Astronomy Tea Talk

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 separation between stellar- and black hole-driven ionization, thereby obscuring both active galactic nuclei demographics and star formation rates. To investigate this, we compute a large suite of photoionization models from Cloudy powered by stellar populations and accreting black holes over a large grid of ages, metallicities, initial mass functions, binarity, ionization parameters, densities, and black hole masses. We use these models to test rest-frame optical diagnostics designed to separate ionizing sources at low redshifts. We show that many line ratios are strongly driven by the ionization parameter and the gas-phase metallicity, 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 metallicity. We show that the OHNO diagram is especially contaminated in the AGN region for stellar models with high log U and low metallicity, consistent with high-redshift observations. We show that these line ratios are most sensitive to the shape of the <54 eV ionizing continuum, and that the ionizing sources derived from optical strong line ratios can be highly degenerate. Finally, we demonstrate that emission lines that trace the >54 eV ionizing continuum help to break the degeneracies present when using strong line diagnostics alone, even in gas conditions consistent with those at high redshifts.