No top-heavy stellar initial mass function needed: the ionizing radiation of GS9422 can be powered by a mixture of AGN 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), 10-30x more top-heavy than a Salpeter IMF (Cameron et al. 2023). 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 piece-wise 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 $\frac{Q_\mathrm{HeII}}{Q_\mathrm{H}} = -1.5$, which can be interpreted as a combination of young, metal-poor stars and a low-luminosity active galactic nucleus (AGN) with $F_{\nu} \propto \lambda ^ {2}$ in a 65%/35% ratio. This suggests a significantly lower nebular continuum contribution to the observed UV flux (24%) than a top-heavy IMF ($\gtrsim80$%), and hence, necessitates a damped Lyman-$\alpha$ absorber (DLA) to explain the continuum turnover bluewards of $\sim1400$ Angstrom. 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.