Reionization after JWST: a photon excess problem?

New James Webb Space Telescope (JWST) observations are revealing the first galaxies to be prolific producers of ionizing photons, which here we argue produces a severe tension between different probes of reionization. For hydrogen reionization to proceed there have to be enough photons to ionize every hydrogen atom. Over the last two decades a consensus has emerged where star-forming galaxies are able to generate these photons, given reasonable values for their ionizing efficiencies 𝜉ion (per unit UV luminosity), escape fractions 𝑓esc, and the “turn-over" magnitude down to which they populate the UV luminosity function (UVLF). However, new JWST observations infer larger values of 𝜉ion for reionization-era galaxies and no turnover down to magnitudes 𝑀UV ≈ −15, dramatically increasing the number of ionizing photons produced at high 𝑧. Simultaneously, recent low-𝑧 studies predict significant escape fractions for those same faint galaxies during reionization. Put together, we show that the galaxies we have directly seen not only can drive reionization, but would end it too quickly. That is, our current galaxy observations would give rise to a cosmic microwave background (CMB) optical depth far in excess of the measurement by the Planck satellite, suggesting an ionizing photon excess during reionization. Considering galaxies down to 𝑀UV ≈ −11, below the current observation limits, only worsens this tension, requiring on average 𝑓esc ≈ 3%, nearly an order of magnitude lower than expected. We discuss possible avenues to ease this tension, including missing astrophysical or observational effects, as well as enhanced recombinations.