Webb Telescope Unravels Cosmic Puzzle: Galaxy Mergers Illuminate Early Universe Mystery

Merging Galaxies Early Universe

The James Webb Space Telescope’s NIRCam has revealed small, faint galaxies merging with larger ones in the early Universe, solving the mystery of detected hydrogen light that should have been obscured. This discovery, alongside advanced simulations, sheds light on galaxy formation and evolution in the Universe’s infancy. Credit: S. Martin-Alvarez

One of the key missions of the NASA/ESA/CSA James Webb Space Telescope is to probe the early Universe. Now, the unmatched resolution and sensitivity of Webb’s NIRCam instrument have revealed, for the first time, what lies in the local environment of galaxies in the very early Universe.

This has solved one of the most puzzling mysteries in astronomy – why astronomers detect light from hydrogen atoms that should have been entirely blocked by the pristine gas that formed after the Big Bang.

Solving an Astronomical Mystery

These new Webb observations have found small, faint objects surrounding the very galaxies that show the ‘inexplicable’ hydrogen emission. In conjunction with state-of-the-art simulations of galaxies in the early Universe, the observations have shown that the chaotic merging of these neighboring galaxies is the source of this hydrogen emission.

Light travels at a finite speed (300,000 km a second), which means that the further away a galaxy is, the longer it has taken the light from it to reach our Solar System. As a result, not only do observations of the most distant galaxies probe the far reaches of the Universe, but they also allow us to study the Universe as it was in the past.

Zooming In on Three Neighboring Galaxies (Webb NIRCam Image)

This image shows the galaxy EGSY8p7, a bright galaxy in the early Universe where light emission is seen from, among other things, excited hydrogen atoms – Lyman-α emission. The galaxy was identified in a field of young galaxies studied by Webb in the CEERS survey. In the bottom two panels, Webb’s high sensitivity picks out this distant galaxy along with its two companion galaxies, where previous observations saw only one larger galaxy in its place. Credit: ESA/Webb, NASA & CSA, S. Finkelstein (UT Austin), M. Bagley (UT Austin), R. Larson (UT Austin), A. Pagan (STScI), C. Witten, M. Zamani (ESA/Webb)

Webb’s Capabilities and Early Galaxy Observations

In order to study the very early Universe, astronomers require exceptionally powerful telescopes that are capable of observing very distant – and therefore very faint – galaxies. One of Webb’s key capabilities is its ability to observe those very distant galaxies, and hence to probe the early history of the Universe. An international team of astronomers has put Webb’s amazing capacity to excellent use in solving a long-standing mystery in astronomy.

The very earliest galaxies were sites of vigorous and active star formation, and as such were rich sources of a type of light emitted by hydrogen atoms called Lyman-α emission.[1]

However, during the epoch of reionization[2] an immense amount of neutral hydrogen gas surrounded these areas of active star formation (also known as stellar nurseries). Furthermore, the space between galaxies was filled by more of this neutral gas than is the case today. The gas can very effectively absorb and scatter this kind of hydrogen emission,[3] so astronomers have long predicted that the abundant Lyman-α emission released in the very early Universe should not be observable today.

This theory has not always stood up to scrutiny, however, as examples of very early hydrogen emission have previously been observed by astronomers. This has presented a mystery: how is it that this hydrogen emission – which should have long since been absorbed or scattered – is being observed? Researcher at the University of Cambridge and principal investigator on the new study Callum Witten elaborates:

One of the most puzzling issues that previous observations presented was the detection of light from hydrogen atoms in the very early Universe, which should have been entirely blocked by the pristine neutral gas that was formed after the Big Bang. Many hypotheses have previously been suggested to explain the great escape of this ‘inexplicable’ emission.”

Close-In View of Three Neighboring Galaxies (Webb NIRCam Image)

This image shows the galaxy EGSY8p7, a bright galaxy in the early Universe where light emission is seen from, among other things, excited hydrogen atoms – Lyman-α emission. Webb’s high sensitivity picks out this distant galaxy along with its two companion galaxies, where previous observations saw only one larger galaxy in its place. Credit: ESA/Webb, NASA & CSA, C. Witten, M. Zamani (ESA/Webb)

Galaxy Mergers and Hydrogen Emission

The team’s breakthrough came thanks to Webb’s extraordinary combination of angular resolution and sensitivity. The observations with Webb’s NIRCam instrument were able to resolve smaller, fainter galaxies that surround the bright galaxies from which the ‘inexplicable’ hydrogen emission had been detected. In other words, the surroundings of these galaxies appear to be a much busier place than we previously thought, filled with small, faint galaxies. Crucially, these smaller galaxies were interacting and merging with one another, and Webb has revealed that galaxy mergers play an important role in explaining the mystery emission from the earliest galaxies. Sergio Martin-Alvarez, team member from Stanford University, adds:

“Where Hubble was seeing only a large galaxy, Webb sees a cluster of smaller interacting galaxies, and this revelation has had a huge impact on our understanding of the unexpected hydrogen emission from some of the first galaxies.”

The Azahar simulations shown in this video are the result of a collaboration between Stanford University and the University of Cambridge, generated in the Cosma supercomputers from the DIRAC UK HPC facilities. Credit: S. Martin-Alvarez

The team then used state-of-the-art computer simulations (a sample of which is highlighted in the video above) to explore the physical processes that might explain their results. They found that the rapid build-up of stellar mass through galaxy mergers both drove strong hydrogen emission and facilitated the escape of that radiation via channels cleared of the abundant neutral gas. So the high merger rate of the previously unobserved smaller galaxies presented a compelling solution to the long-standing puzzle of the ‘inexplicable’ early hydrogen emission.

Future Research and Understanding Galaxy Evolution

The team is planning follow up observations with galaxies at various stages of merging, in order to continue to develop their understanding of how the hydrogen emission is ejected from these changing systems. Ultimately, this will enable them to improve our understanding of galaxy evolution.

These findings were published on January 18 in Nature Astronomy.


  1. Lyman-α emission is light emitted at a wavelength of 121.567 nanometres when the electron in an excited hydrogen atom drops from an excited state in the n = 2 orbital down to its ground state n = 1 (the lowest energy state the atom can have). Quantum physics dictates that electrons can only exist in very specific energy states, and this means that certain energy transitions – such as when the electron in a hydrogen atom drops from orbital n = 2 to n = 1 – can be identified by the wavelength of the light emitted during that transition. Lyman-α emission is important in many branches of astronomy, partly because hydrogen is so abundant in the Universe, and also because hydrogen is typically excited by energetic processes such as ongoing active star formation. Accordingly, Lyman-α emission can be used as a sign that active star formation is taking place.
  2. The epoch of reionization was a very early stage in the Universe’s history that took place after recombination (the first stage following the Big Bang). During recombination, the Universe cooled enough that electrons and protons began to combine to form neutral hydrogen atoms. During reionization, denser clouds of gas started to form, creating stars and eventually entire galaxies whose light gradually reionized the hydrogen gas.
  3. Neutral hydrogen gas is made of hydrogen atoms that are in the lowest energy state they can be, each with their electron in orbital n = 1. Since the light emitted by a hydrogen atom during Lyman-α emission carries the energy of the atomic transition from orbital n = 2 down to n = 1, when it strikes a neutral hydrogen atom, it has exactly the right amount of energy to ionize the atom and take its electron up to the next available orbital. This means the neutral gas absorbs and blocks Lyman-α emission very easily.

Reference: “Deciphering Lyman-α emission deep into the epoch of reionization” by Callum Witten, Nicolas Laporte, Sergio Martin-Alvarez, Debora Sijacki, Yuxuan Yuan, Martin G. Haehnelt, William M. Baker, James S. Dunlop, Richard S. Ellis, Norman A. Grogin, Garth Illingworth, Harley Katz, Anton M. Koekemoer, Daniel Magee, Roberto Maiolino, William McClymont, Pablo G. Pérez-González, Dávid Puskás, Guido Roberts-Borsani, Paola Santini and Charlotte Simmonds, 18 January 2024, Nature Astronomy.
DOI: 10.1038/s41550-023-02179-3

More information

The James Webb Space Telescope 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).

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