Towards Reconciling Reionization with JWST: The Role of Bright Galaxies and Strong Feedback

TL;DR

The paper tackles the tension between JWST-derived high-redshift UV luminosity functions and reionization constraints by integrating a radiative-transfer–informed, semi-analytic source model with an MCMC calibration framework to jointly fit JWST UVLF/ρUV and reionization observables such as $\dot{N}_{\rm ion}$, $x_{\rm HI}$, and $\tau$. It demonstrates two viable regimes: a weak-feedback scenario where faint galaxies drive reionization but underproduce the JWST UVLF at $z>9$, and a strong-feedback scenario where bright galaxies reproduce the UVLF at $z\ge10$ yet overestimate brightness at $z<9$, yielding an extended reionization history ($z\sim16$–$6$) with a $\tau$ in agreement with CMB measurements at $2\sigma$. This extended reionization helps alleviate the photon-budget crisis while ensuring reionization completes by $z\sim6$ in both cases. The study highlights the critical role of feedback strength and the evolving ionizing emissivity of different galaxy populations in reconciling early JWST observations with cosmic reionization timelines, using a jointly constrained, physically motivated framework.

Abstract

The elevated UV luminosity functions (UVLF) from recent James Webb Space Telescope (JWST) have challenged the viability of existing theoretical models. To address this, we use a semi-analytical framework -- which couples a physically motivated source model derived from radiative-transfer hydrodynamic simulations of reionization with a Markov Chain Monte Carlo sampler -- to perform a joint calibration to JWST galaxy surveys (UVLF, $φ_{\rm UV}$ and UV luminosity density, $ρ_{\rm UV}$) and reionization-era observables (ionizing emissivity, $\dot{N}_{\rm ion}$, neutral hydrogen fraction, $x_{\rm HI}$, and Thomson optical depth, $τ$). We find that models with weak feedback and a higher contribution from faint galaxies reproduce the reionization observables but struggle to match the elevated JWST UVLF at $z > 9$. In contrast, models with stronger feedback (i.e., rapid redshift evolution) and a higher contribution from bright galaxies successfully reproduce JWST UVLF at $z \geq 10$, but over-estimate the bright end at $z < 9$. The strong-feedback model constrained by JWST UVLF predicts a more gradual and extended reionization history, as opposed to the sudden reionization seen in the weak-feedback models. This extended nature of reionization ($z\sim 16$ - $6$) yields an optical depth consistent (at 2-$σ$) with the Cosmic Microwave Background (CMB) constraint, thereby alleviating the photon-budget crisis. In both scenarios, reionization is complete by $z \sim 6$, consistent with current data. Our analysis highlights the importance of accurately modeling feedback and ionizing emissivities from different source populations during the first billion years after the Big Bang.