Les Lumières: probing the cosmic Epoch of Reionization with high-redshift quasars

Kist, T. (2026)

Doctoral Thesis

Around 300 years ago – when individuals finally emerged from their self-imposed immaturity – the Age of Enlightenment was the final nail in the coffin of the medieval dark ages. Significantly earlier, around 13 billion years ago, stars and galaxies – the first astrophysical individuals – started forming and initiated a cosmic Age of Enlightenment: the Epoch of Reionisation.

Quasars are particularly powerful tools to constrain the timing of this early cosmic era as any amount of neutral hydrogen in the surrounding intergalactic medium (IGM) attenuates the Lyman-alpha line seen in their spectra. By working with a state-of-the-art Bayesian inference pipeline, built to reconstruct the exact strength of this Lyman-alpha damping wing, and thus constraining the Universe's reionisation state at the quasar's redshift, I perform a thorough quantification of all associated modelling uncertainties.

I also put forward a new set of summary statistics that allows for a more optimal extraction of all the cosmological information carried by the imprint – not only about the global timing of the reionisation era but also its topology. By incorporating these statistics into our inference pipeline, I present the first constraints on the local ionisation topology in front of two of the highest-redshift (z ~ 7.5) quasars known to date.

• Reionization
• Cosmic dawn
• Intergalactic medium
• Quasars
• Supermassive black holes
• Spectroscopy
• Radiative transfer
• Statistical inference
• Hamiltonian Monte Carlo
• Differentiable programming

• All authors: Kist, T.
• Supervisor: Hennawi, J.F.; Schaye, J.
• Committee: Snellen, I.A.G.; Kuijken, K.H.; Sellentin, E.; Davies, F.B. Davies; Haehnelt, M.G.; Hoekstra, H.
• Qualification: Doctor (dr.)
• Awarding Institution: Leiden Observatory, Faculty of Science, Leiden University
• Date: 2026-07-03