weic2405 — Science Release

Webb finds dwarf galaxies reionised the Universe

28 February 2024

Using the unprecedented capabilities of the NASA/ESA/CSA James Webb Space Telescope, an international team of scientists have obtained the first spectroscopic observations of the faintest galaxies during the first billion years of the Universe. These findings help answer a longstanding question for astronomers: what sources caused the reionisation of the Universe? These news results have effectively demonstrated that small dwarf galaxies are the likely producers of prodigious amounts of energetic radiation.

Researching the evolution of the early Universe is an important aspect of modern astronomy. Much remains to be understood about the time in the Universe’s early history known as the era of reionisation [1]. It was a period of darkness without any stars or galaxies, filled with a dense fog of hydrogen gas, until the first stars ionised the gas around them and light began to travel through. Astronomers have spent decades trying to identify the sources that emitted radiation powerful enough to gradually clear away this hydrogen fog that blanketed the early Universe.

The Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization (UNCOVER) programme (#2561) consists of both imaging and spectroscopic observations of the lensing cluster Abell 2744. An international team of astronomers used gravitational lensing by this target, also known as Pandora’s Cluster, to investigate the sources of the Universe’s period of reionisation. Gravitational lensing [2] magnifies and distorts the appearance of distant galaxies, so they look very different from those in the foreground. The galaxy cluster 'lens' is so massive that it warps the fabric of space itself, so much so that light from distant galaxies that passes through the warped space also takes on a warped appearance. The magnification effect allowed the team to study very distant sources of light beyond Abell 2744, revealing eight extremely faint galaxies that would otherwise be undetectable, even to Webb.

The team found that these faint galaxies are immense producers of ionising radiation, at levels that are four times larger than what was previously assumed. This means that most of the photons that reionised the Universe likely came from these dwarf galaxies.

This discovery unveils the crucial role played by ultra-faint galaxies in the early Universe's evolution,” said team member Iryna Chemerynska of the Institut d’Astrophysique de Paris in France. “They produce ionising photons that transform neutral hydrogen into ionised plasma during cosmic reionisation. It highlights the importance of understanding low-mass galaxies in shaping the Universe's history.

“These cosmic powerhouses collectively emit more than enough energy to get the job done,” added team leader Hakim Atek, Institut d’Astrophysique de Paris, CNRS, Sorbonne Université, France, and lead author of the paper describing this result. “Despite their tiny size, these low-mass galaxies are prolific producers of energetic radiation, and their abundance during this period is so substantial that their collective influence can transform the entire state of the Universe.”

To arrive at this conclusion, the team first combined ultra-deep Webb imaging data with ancillary imaging of Abell 2744 from the NASA/ESA Hubble Space Telescope in order to select extremely faint galaxy candidates in the epoch of reionisation. This was followed by spectroscopy with Webb’s Near-InfraRed Spectrograph (NIRSpec). The instrument’s Multi-Shutter Assembly was used to obtain multi-object spectroscopy of these faint galaxies. This is the first time scientists have robustly measured the number density of these faint galaxies, and they have successfully confirmed that they are the most abundant population during the epoch of reionisation. This also marks the first time that the ionising power of these galaxies has been measured, enabling the astronomers to determine that they are producing sufficient energetic radiation to ionise the early Universe.

“The incredible sensitivity of NIRSpec combined with the gravitational amplification provided by Abell 2744 enabled us to identify and study these galaxies from the first billion years of the Universe in detail, despite their being over 100 times fainter than our own Milky Way,” continued Atek.

In an upcoming Webb observing programme, termed GLIMPSE, scientists will obtain the deepest observations ever on the sky. By targeting another galaxy cluster, named Abell S1063, even fainter galaxies during the epoch of reionisation will be identified in order to verify whether this population is representative of the large-scale distribution of galaxies. As these new results are based on observations obtained in one field, the team notes that the ionising properties of faint galaxies can appear differently if they reside in over-dense regions. Additional observations in an independent field will therefore provide further insights to help verify these conclusions. The GLIMPSE observations will also help astronomers probe the period known as Cosmic Dawn, when the Universe was only a few million years old, to develop our understanding of the emergence of the first galaxies. 

These results have been published today in the journal Nature.


[1] Theory predicts that the first stars were 30 to 300 times as massive as our Sun and millions of times as bright, burning for only a few million years before exploding as supernovae. The energetic ultraviolet light from these first stars was capable of splitting hydrogen atoms back into electrons and protons (or ionising them). This era, from the end of the dark ages to when the Universe was around a billion years old, is known as the epoch of reionisation. This is the period when most of the neutral hydrogen was reionised by the increasing radiation from the first massive stars. Reionisation is an important phenomenon in our Universe's history as it presents one of the few means by which we can (indirectly) study these earliest stars and galaxies. 

[2] Gravitational lensing occurs when a massive celestial body — such as a galaxy cluster — causes a sufficient curvature of spacetime for the path of light around it to be visibly bent, as if by a lens. The body causing the light to curve is accordingly called a gravitational lens. According to Einstein’s general theory of relativity, time and space are fused together in a quantity known as spacetime. Within this theory, massive objects cause spacetime to curve, and gravity is simply the curvature of spacetime. As light travels through spacetime, the theory predicts that the path taken by the light will also be curved by an object’s mass. Gravitational lensing is a dramatic and observable example of Einstein's theory in action. Extremely massive celestial bodies such as galaxy clusters cause spacetime to be significantly curved. In other words, they act as gravitational lenses. When light from a more distant light source passes by a gravitational lens, the path of the light is curved, and a distorted image of the distant object results.

More information

Webb is the largest, most powerful telescope ever launched into space. Under an international collaboration agreement, ESA provided the telescope’s launch service, using the Ariane 5 launch vehicle. Working with partners, ESA was responsible for the development and qualification of Ariane 5 adaptations for the Webb mission and for the procurement of the launch service by Arianespace. ESA also provided the workhorse spectrograph NIRSpec and 50% of the mid-infrared instrument MIRI, which was designed and built by a consortium of nationally funded European Institutes (The MIRI European Consortium) in partnership with JPL and the University of Arizona.

Webb is an international partnership between NASA, ESA and the Canadian Space Agency (CSA).



Hakim Atek
Institut d’Astrophysique de Paris, CNRS, Sorbonne Université, France
Email: [email protected]

Bethany Downer
ESA/Webb Chief Science Communications Officer
Email: [email protected]

Ninja Menning
ESA Newsroom and Media Relations Office
Email: [email protected]

About the Release

Release No.:weic2405


Pandora’s Cluster (Abell 2744)
Pandora’s Cluster (Abell 2744)

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