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

TL;DR

This paper introduces PHOTONIOn, a new photometry-based inference pipeline that reliably estimates the ionizing photon output of high-redshift galaxies, shedding light on their role in cosmic reionization.

Contribution

The paper presents PHOTONIOn, an implicit likelihood inference method trained on mock data, which accurately predicts ionizing luminosity from photometry, outperforming traditional SED-fitting approaches.

Findings

PHOTONIOn reliably infers ionizing photon rates from photometry.

High-redshift galaxies can reionize the universe by z~5.3 without exotic sources.

UV-faint galaxies significantly contribute to reionization.

Abstract

JWST is providing constraints on the history of reionization owing to its ability to detect faint galaxies at $z\gg6$. Modeling this history requires understanding both the ionizing photon production rate ($\xi_{\rm ion}$) and the fraction of those photons that escape into the intergalactic medium ($f_{\rm esc}$). Observational estimates of these quantities generally rely on spectroscopy for which large samples with well-defined selection functions are limited. To overcome this challenge, we present and release an implicit likelihood inference pipeline, PHOTONIOn, trained on mock photometry to predict the escaped ionizing luminosity of individual galaxies ($\dot{N}_{\rm ion}$) based on photometric magnitudes and redshifts. We show that PHOTONIOn is able to reliably infer $\dot{N}_{\rm ion}$ from photometry. This is in contrast to traditional SED-fitting approaches which rely on $f_{\rm esc}$ prescriptions that often over-predict $\dot{N}_{\rm ion}$ for LyC-dim galaxies, even when given access to spectroscopic data. We have deployed PHOTONIOn on a sample of 4,559 high-redshift galaxies from the JADES Deep survey, finding gentle redshift evolutions of $\log_{10}(\dot{N}_{\rm ion}) = (0.08\pm0.01)z + (51.60\pm0.06)$ and $\log_{10}(f_{\rm esc}\xi_{\rm ion}) = (0.07\pm0.01)z + (24.12\pm0.07)$. Late-time values for the ionizing photon production rate density are consistent with theoretical models and observations. We measure the evolution of the IGM ionized fraction to find that observed populations of star-forming galaxies are capable of driving reionization in GOODS-S to completion by $z\sim 5.3$ without the need for AGN or other exotic sources, consistent with other studies of the same field. The $20\%$ of UV-brightest galaxies ($M_{\rm UV}<-18.5$) reionize $35\%$ of the survey volume, demonstrating that UV faint LyC emitters are crucial for reionization.