The First Galaxies

This moment, when the Universe was first bathed in starlight, is sometimes referred to as “Cosmic Dawn” or “First Light”. It is thought such early galaxies, whose first stars contained no chemical elements other than the hydrogen and helium nuclei synthesised in the Big Bang, produced copious amounts of ultraviolet radiation. This radiation photo-ionised the hydrogen in deep space - a process called “Cosmic Reionisation” - over the first billion years of cosmic history (corresponding to a redshift 6), yielding the fully ionised Universe we live in today. Our group is using both space and ground-based telescopes, supported with numerical simulations, to explore this largely uncharted era and pinpoint the properties of the earliest generations of galaxies. Key questions include: when did the first stars and galaxies form, how quickly did they assemble their stellar and chemical contents, did they have black holes, and which of these sources were responsible for the reionisation process?

Since its launch in December 2021, the James Webb Space Telescope (JWST) has been transforming our understanding of the earliest galaxies. JWST’s unprecedented infrared sensitivity has enabled the detection and study of galaxies out to redshifts of z~15-20, corresponding to just ~200-300 million years after the Big Bang. These observations have revealed an early universe far more diverse and remarkable than predicted, including an overabundance of exceptionally-luminous galaxies at z>10, the early birth and rapid growth of supermassive black holes, peculiar chemical abundances from potentially exotic stellar populations, and large-scale ionised bubbles in the intergalactic medium.

Deciphering these observational puzzles requires both photometric and spectroscopic measurements of the most distant sources, both intrinsically bright and faint (e.g., using gravitational lensing), with which to uncover the underlying physical mechanisms powering these remarkable properties. At UCL, we are actively leading JWST and ALMA observations of z>6 galaxies through multiple programs, to characterise their cosmic number densities, the ages and masses of their stellar populations, their chemical abundances and synthesis pathways, supermassive black hole activity, and ionising capabilities. Interpreting this wealth of observational data increasingly relies on new and accurate theoretical and simulated frameworks, whose models of galaxy number densities, star formation processes, radiative transfer, and chemical enrichment provide an essential benchmark for understanding what we observe. At UCL, we are involved in DiRAC programs and collaborations to generate state-of-the-art simulations of the earliest galaxies, allowing us to directly compare theoretical predictions with the remarkable discoveries being uncovered by JWST.

With the aid of novel machine learning techniques and the latest simulations, the selection and characterisation methods developed with JWST will be applied to upcoming facilities including the Euclid space telescope, the Extremely Large Telescope (ELT), and the Square Kilometre Array (SKA). Euclid will provide unprecedented statistical samples of luminous galaxies at high-redshift, while the ELT will deliver high-resolution spectroscopic follow-up of many such sources. Combining these with SKA correlations of of neutral hydrogen during the epoch of reionisation, these multi-wavelength observations will provide a comprehensive view of how the universe transformed from darkness to light during its first billion years, with UCL involvement at the forefront.

If you are interested in getting involved, or learning about potential high-redshift projects, please contact Dr. Guido Roberts-Borsani (g.robertsborsani@ucl.ac.uk) or Prof. Richard Ellis (richard.ellis@ucl.ac.uk).