Date: 10/13/2024
Harvard ADS: Reionization after JWST: a photon budget crisis?
Paper 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 \xi _{\rm ion} (per unit ultraviolet luminosity), and escape fractions f_{\rm esc}. However, some new JWST observations infer high values of \xi _{\rm ion} during reionization and an enhanced abundance of earlier (z\gtrsim 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_{\rm UV} < -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-\alpha forest. Considering galaxies down to M_{\rm UV}~ -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.
