James Webb Space Telescope Feed Post

Arxiv: Reionization after JWST: a photon budget crisis? Published: 4/10/2024 9:00:00 PM Updated: 4/10/2024 9:00:00 PM

Paper abstract: New James Webb Space Telescope (JWST) observations are revealing the firstgalaxies to be prolific producers of ionizing photons, which we argue givesrise to a tension between different probes of reionization. For hydrogenreionization to proceed there must be enough ionizing photons for all of thehydrogen atoms, including their recombinations. Over the last two decades aconsensus has emerged where star-forming galaxies are able to generate thesephotons, given reasonable values for their number densities, ionizingefficiencies \xi_{\rm ion} (per unit UV luminosity), and escape fractionsf_{\rm esc}. However, new JWST observations infer high values of \xi_{\rmion} during reionization and an enhanced abundance of earlier (z\gtrsim 9)galaxies, dramatically increasing the number of ionizing photons produced athigh z. Simultaneously, recent low-z studies predict significant escapefractions for faint reionization-era galaxies. Put together, we show that thegalaxies we have directly observed (M_{\rm UV} < -15) not only can drivereionization, but would end it too early. That is, our current galaxyobservations, taken at face value, imply an excess of ionizing photons and thusa process of reionization in tension with the cosmic microwave background (CMB)and Lyman-\alpha forest. Considering galaxies down to M_{\rm UV}~-11, below current observational limits, only worsens this tension, requiringon average f_{\rm esc}~ 3\%, far lower than expected for early galaxiesfrom post-reionization studies. We discuss possible avenues to resolve thisphoton budget crisis, including missing astrophysical or observationalselection effects, as well as enhanced recombinations.