No Top-heavy Stellar Initial Mass Function Needed: The Ionizing Radiation of GS9422 Can Be Powered by a Mixture of an Active Galactic Nucleus and Stars

Abstract

JWST is producing high-quality rest-frame optical and UV spectra of faint galaxies at z > 4 for the first time, challenging models of galaxy and stellar populations. One galaxy recently observed at z = 5.943, GS9422, has nebular line and UV continuum emission that appears to require a high ionizing photon production efficiency. This has been explained with an exotic stellar initial mass function (IMF; Cameron et al. 2023a), 10–30x more top-heavy than a Salpeter IMF. Here we suggest an alternate explanation to this exotic IMF. We use a new flexible neural net emulator for CLOUDY, Cue , to infer the shape of the ionizing spectrum directly from the observed emission line fluxes. By describing the ionizing spectrum with a piecewise power law, Cue is agnostic to the source of the ionizing photons. Cue finds that the ionizing radiation from GS9422 can be approximated by a double power law characterized by $\tfrac{{Q}_{{\rm{HeII}}}}{{Q}_{{\rm{H}}}}=-1.5$ , which can be interpreted as a combination of young, metal-poor stars and a low-luminosity active galactic nucleus with F _ν ∝ λ ^2 in a 65%/35% ratio. This suggests a significantly lower nebular continuum contribution to the observed UV flux (24%) than a top-heavy IMF (≳80%), and hence, necessitates a damped Ly α absorber to explain the continuum turnover bluewards of ∼1400 Å. While current data cannot rule out either scenario, given the immense impact the proposed top-heavy IMF would have on models of galaxy formation, it is important to propose viable alternative explanations and to further investigate the nature of peculiar high- z nebular emitters.

Keywords

• Photoionization
• AGN host galaxies
• Interstellar medium
• High-redshift galaxies