Reionization after JWST: A photon budget crisis?

ABSTRACT New JWST observations are revealing the first galaxies to be prolific producers of ionizing photons, which we argue gives rise to a tension between different probes of reionization. Over the last two decades, a consensus has emerged where star-forming galaxies are able to generate enough photons to drive reionization, given reasonable values for their number densities, ionizing efficiencies ₈₎₍ (per unit ultraviolet luminosity), and escape fractions f ₄ₒ₂. However, some new JWST observations infer high values of ₈₎₍ during reionization and an enhanced abundance of earlier (z 9) galaxies, dramatically increasing the number of ionizing photons produced at high z. Simultaneously, recent low-z studies predict significant escape fractions for faint reionization-era galaxies. Put together, we show that the galaxies we have directly observed (M ₔₕ -15) not only can drive reionization, but would end it too early. That is, our current galaxy observations, taken at face value, imply an excess of ionizing photons and thus a process of reionization in tension with the cosmic microwave background and Lyman- forest. Considering galaxies down to M ₔₕ -11, below current observational limits, only worsens this tension. We discuss possible avenues to resolve this photon budget crisis, including systematics in either theory or observations.