Unravelling the story of cosmic reionization: learning from high-redshift Lyman continuum emitters

by Dr Ilias Goovaerts (JHU/STScI, MD USA)

Wednesday 17 Jun 2026, 13:30 → 14:30 Europe/Stockholm

FC61 (AlbaNova Main Building)

The epoch of cosmic reionisation was the last phase transition the Universe underwent, changing all the hydrogen in the intergalactic medium from neutral to ionized. This transition had fundamental impacts on galaxy evolution, as well as being the era during which the first galaxies were formed. Significant open questions remain on how this change happened, and which ionising sources were responsible. Given the huge advances made by JWST into the understanding of galaxies during this epoch, resolving the growth of these galaxies with their impact on the intergalactic medium remains a central effort of modern astrophysics.

I will give an overview of reionisation, our current understanding of how galaxies impacted this process and what is still unknown. I will focus on the recent efforts to build samples of high-redshift Lyman continuum emitters — galaxies that emit ionising photons — and what we can learn from the most extreme, and high-redshift emitters detected to date. I’ll present new results on Ion3 and MXDFz4.4, the highest redshift directly-detected Lyman continuum emitters and discuss what these galaxies tell us about how early ionising sources shaped their environment. I’ll discuss their place in the hunt to understand ionising emission at the highest redshifts possible and how these extreme galaxies may be different to low-redshift analogues.

Finally I’ll discuss ongoing and future efforts to detect and characterise ionizing emission, ranging from crucial work in HST’s final years (hopefully still many!) to transformative advances targeted with the next generation of 30+ meter class ground-based telescopes such as the ELT, and the forthcoming Habitable Worlds Observatory.