Reionization driven by the few: the ionizing budget of galaxies at z=5-10 from JWST/NIRSpec

The sources responsible for the Epoch of Reionization (EoR), the last major phase transition of the Universe, remain highly elusive. While JWST has begun to illuminate the properties of early galaxies, the direct detection of their ionizing photons is virtually impossible due to the neutral intergalactic medium (IGM) at z>6. A direct estimate of the escape fraction of ionizing photons (fesc) is thus not possible. However, the escape fraction is encoded in spectral features that trace a low opacity to ionizing photons, namely the reduction of nebular emission with increasing fesc. Here, we exploit the large archive of JWST/NIRSpec spectra at 5<z<10 from the DAWN JWST Archive to provide new constraints on the ionizing emissivity of EoR galaxies and on the timeline of reionization. Through detailed spectral fitting of 1428 galaxies with a picket-fence model for the escape of ionizing photons, we derive posteriors of fesc together with the effective ionizing output. This approach yields the escaping ionizing output of each galaxy while bypassing the usual decomposition into the ionizing photon production efficiency and fesc. Combining these measurements with UV luminosity functions (LFs), we derive the hydrogen ionizing LFs, resulting in an observationally-derived ionizing photon budget at 5<z<15, integrated down to MUV=-13. Under standard assumptions, our budget yields an evolution of the IGM neutral fraction consistent with independent probes, with reionization ending at z 5.8. Given the evolving shape of the ionizing LF, bright sources (MUV-20) dominate the cosmic ionizing emissivity nion at lower redshift (z=5-6), whereas at z>6 faint galaxies (MUV > -18) contribute roughly equally. Remarkably, a mere 20% of sources, those with fesc>10%, produce 87% of the ionizing photons. Overall, our results support a picture in which a few strongly leaking galaxies drive most of reionization.