Master Thesis Defense by Kimi Cardoso Kreilgaard

Title: Inferring the Distribution of the Ionising Photon Escape Fraction

Abstract: The escape fraction of Lyman continuum photons is a key parameter for understanding how intergalactic hydrogen in the Universe became reionised during the Epoch of Reionisation, but it is mostly unconstrained due to the difficulty of observing ionising photons from distant galaxies. Until now, measurements of the mean escape fraction for the population of galaxies have been made along with measurements of the escape fraction for a small number of individual galaxies. This thesis explores a novel method for uncovering the distribution of escape fractions across a population of 148 star-forming galaxies at z=3.5 from the VANDELS survey. We extend the method of previous work to that of a hierarchical Bayesian inference scheme in an effort to constrain the population parameters that describe the overall distribution of escape fraction. We build an ensemble of mock observations to assess whether it is feasible to produce robust constraints on the population parameters with the sample of 148 real galaxies. We reproduce constraints on the average escape fraction using a previously established single layer inference method, finding that  <f_esc> = 0.07 +0.03 -0.01 (consistent with previous work). Employing a hierarchical Bayesian inference framework with a truncated normal population distribution of escape fractions, we find through the study of mock observations that depending on the underlying escape fraction we need between 300 and 1000 spectroscopically confirmed galaxies to produce meaningful constraints on the population parameters. Future work should focus on including photometric galaxies in the data to be able to use the method with current existing data.

Supervisor:

• Charlotte Mason, University of Copenhagen, Niels Bohr Institute

Co-Supervisor:

• Fergus Cullen, University of Edinburgh

Censor:

• Hans Kjeldsen, Aarhus University