Capturing Small-Scale Reionization Physics: A Sub-Grid Model for Photon Sinks with SCRIPT

Abstract

The epoch of reionization represents a major phase transition in cosmic history, during which the first luminous sources ionized the intergalactic medium (IGM). However, the small-scale physics governing ionizing photon sinks - particularly the interplay between recombinations, photon propagation, and self-shielded regions - remains poorly understood. Accurately modeling these processes requires a framework that self-consistently links ionizing emissivity, the clumping factor, mean free path, and photoionization rate. In this work, we extend the photon-conserving semi-numerical framework, SCRIPT, by introducing a self-consistent sub-grid model that dynamically connects these quantities to the underlying density field, enabling a more realistic treatment of inhomogeneous recombinations and photon sinks. We validate our model against a comprehensive set of observational constraints, including the UV luminosity function from HST and JWST, CMB optical depth from Planck, and Lyman-$\alpha$ forest measurements of the IGM temperature, photoionization rate, and mean free path. Our fiducial model also successfully reproduces Lyman-$\alpha$ opacity fluctuations, reinforcing its ability to capture large-scale inhomogeneities in the reionization process. Notably, we demonstrate that traditionally independent parameters, such as the clumping factor and mean free path, are strongly correlated, with implications for the timing, morphology, and thermal evolution of reionization. Looking ahead, we will extend this framework to include machine learning-based parameter inference. With upcoming 21cm experiments poised to provide unprecedented insights, SCRIPT offers a powerful computational tool for interpreting high-redshift observations and refining our understanding of the last major phase transition in the universe.