Inferring the Ionizing Photon Contributions of High-Redshift Galaxies to Reionization with JWST NIRCam Photometry

JWST observations have the potential to provide unprecedented constraints on the history of reionization and the sources responsible for the ionizing photons due to the detection of large populations of faint galaxies at $z\gg6$.

Modelling reionization requires knowing both the number of ionizing photons that are produced by galaxies and the fraction of those photons that escape into the intergalactic medium.

Observational estimates of these values generally rely on spectroscopy for which large samples with well-defined selection functions remain limited.

To overcome this challenge, we present an implicit likelihood inference (ILI) pipeline trained on mock photometry to predict the escaped ionizing luminosity of individual galaxies ($\dot{n}_{\rm ion}$) based on photometric magnitudes and redshifts.

Compared to traditional SED-fitting methods, the new ILI pipeline is consistently more accurate and significantly faster.

We deploy the method on a sample of 4,559 high-redshift galaxies from the JADES Deep survey, finding a gentle redshift evolution of $\log_{10}(\dot{n}_{\rm ion}) = (0.08\pm0.01)z + (51.60\pm0.06)$, with late-time values for $\dot{N}_{\rm ion}$ consistent with theoretical models and observations.

We measure the evolution of the volume-averaged ionized fraction and optical depth to find that observed populations of star-forming galaxies are capable of driving reionization to completion at $z\sim 5.3$ without the need for AGN or other exotic sources.

The $20\%$ of UV-brightest galaxies ($M_{\rm UV}<-18.5$) in our sample can reionize only $\sim30\%$ of the survey volume, demonstrating that faint LyC emitters are crucial for reionization.

;Comment: 17 pages, 11 figures, 2 tables, submitted to MNRAS