Tracing the memory of the IGM

by Dr Arghyadeep Basu (Centre de Recherche Astrophysique de Lyon (CRAL))

Wednesday 21 Jan 2026, 14:30 → 15:30 Asia/Kolkata

A-304 and on Zoom

Understanding how the first galaxies formed and drove cosmic reionization remains one of the central challenges in modern cosmology. Feedback processes regulate star formation in early galaxies, shaping the ultraviolet luminosity function (UV LF) and potentially explaining the reported excess of bright galaxies at very high redshift (z ≳ 10). At the same time, the nature of ionizing sources leaves long-lasting imprints on the intergalactic medium (IGM), which can be probed at much later times through observations such as the Lyman-α forest. Linking these galaxy-scale and IGM-scale signatures requires accurate modeling of the properties of early galaxies (ie. SEDs). However, cosmic reionization is fundamentally difficult to study because the sources themselves are not observed directly; instead, we infer their properties from indirect signatures at later time. While simulations describe galaxies and the IGM in terms of physical quantities such as star formation, feedback, and ionizing radiation, observations like JWST programs measure only photons, in the form of fluxes, images, colors, magnitudes, and spectra. Recent JWST surveys, including the GLIMPSE program, have pushed to unprecedented depths, revealing extremely faint galaxies at very high redshift and opening new opportunities to test models of early galaxy formation, while also highlighting the gap between theoretical predictions and observable quantities. On the IGM side, reionization is constrained indirectly through probes such as Lyman-α absorption, effective optical depths, temperature measurements, and the redshifted 21 cm signal. In this talk, I will give a general overview of how mock observations help bridge this gap by transforming simulation outputs into realistic synthetic observations that can be compared directly with data. I will introduce the basic idea behind mock observations and illustrate how they enable connections between galaxy properties and observables and a unified interpretation of early JWST results and IGM measurements in the context of cosmic reionization.