Reionization after JWST: a photon budget crisis?

Abstract

New James Webb Space Telescope (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 ${\xi }_{\mathrm{ion}}$ (per unit UV luminosity), and escape fractions $f_{\mathrm{esc}}$. However, some new JWST observations infer high values of ${\xi }_{\mathrm{ion}}$ 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 ($M_{\mathrm{UV}}<-15$) 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 (CMB) and Lyman-$\alpha$ forest. Considering galaxies down to $M_{\mathrm{UV}}\approx -11$, 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.

arXiv comment6+3 pages, 3+2 figures, 1 table, updated to match accepted version

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