Dartmouth Events

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 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. 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. We also discuss the implications of >54 eV emission in constraining black hole formation pathways and in constraining the geometry of Little Red Dots. We finish by showing future developments with surveys from Roman and JWST.

Hosted By Research Associate Jonathan Cohn 

Zoom Link: https://dartmouth.zoom.us/j/93069806358