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Department of Physics

The Cavendish Laboratory
Swan Nebula

What made the cosmos go from darkness to radiant? Researchers at the University of Cambridge have gone further in time than ever before to uncover crucial insights on the role of younger, low-mass galaxies with bursty star formation in this transformation, known at the epoch of Reionization.


Studying a representative sample of low-mass galaxies 0.5-1.5 billion years after the Big Bang, they have established that these galaxies could be efficient enough in generating ionising photons, responsible for the ionisation – or illumination - of the Universe.

“Imagine a time around 400 thousand years after the Big Bang when the cosmos shifted from darkness to the radiant universe we know today,” said Charlotte Simmonds, Postdoctoral researcher at the Cavendish Laboratory, University of Cambridge. “This transformation, known as the epoch of Reionization (EoR), was driven by the emergence of the first stars and galaxies. Understanding these galaxies and their role in illuminating the Universe has been a big challenge in modern cosmology and researchers continue to persist in their pursuit of finding an answer.”

In this new study, published in Oxford Academic, the scientists from the Cavendish Laboratory have examined 677 galaxies with the help of data received from James Webb Space Telescope (JWST), particularly the Advanced Deep Extragalactic Survey (JADES).

They were able to assemble the biggest sample to date in the field. Going further back in time than ever before, spanning 0.5-1.5 billion years post-Big Bang, the study focused on the galaxies' ionising photon production efficiency - or ξion.

“ξion is essentially a measure of how good galaxies are at producing photons with enough energy to kick out an electron from a neutral hydrogen atom,” said Simmonds. “This phenomenon is instrumental in illuminating the Universe as we see today.”

The study reveals that the further we go back in time, the more efficient galaxies are at producing ionising radiation. Younger stars are hotter and produce more ionising radiation, therefore, a bursty star formation in a young galaxy leads to a higher ξion.

Crucially, the increase in ξion is driven by the mass of the galaxies. Low-mass galaxies, characterised by their youth and bursty star formation, form stars in bursts. This is mainly driven by Supernova explosions, which expel gas, temporarily halting star formation until the medium cools and settles, leading to a higher ξion.

The researchers used two approaches to estimate ξion. First, they looked at specific atomic hydrogen and oxygen transitions in their sample using JWST data. Then, they employed a powerful software to analyse ξion values along with other galaxy properties such as mass and star formation rate. The combination of these methods allowed them to give an explanation to the observed increase of ξion as we go deeper into the early Universe.

“Although this is not the first research to study ξion as a function of time,  this is the first-time we were able to assemble the biggest sample to date in the field," said Simmonds. “This marks a crucial chapter in our cosmic narrative, bringing us closer to unravelling the mysteries of the Universe's transformation from darkness to light billions of years after the Big Bang.”

Next, the researchers plan to conduct a similar study without pre-selecting specific types of galaxies. Their aim is to unpick the contributions of different galaxy populations to the reionisation process, and better understand the sources that illuminated the Universe during its cosmic dawn.


Simmonds, C. et al. 'Low-mass bursty galaxies in JADES efficiently produce ionising photons and could represent the main drivers of reionisationMonthly Notices of the Royal Astronomical Society, 2023;, stad3605. DOI: 10.1093/mnras/stad3605

Image: Swan Nebula – M17, The Swan or Omega Nebula (named after its shape) is an HII region (composed of ionised hydrogen) with a cluster of young stars embedded in it. This nebula is one of the brightest and most massive star-forming regions in the Milky Way galaxy. The young blue stars flood the nebula with strong ultraviolet radiation, ionising the hydrogen in the surrounding nebulosity. This was one of the top entries of Cavendish Photography Competition 2022. | Credit: David Puskas, PhD student, Cavendish Laboratory