Galaxy mergers solve early Universe mystery

A team of astronomers, led by the University of Cambridge, has used the NASA/ESA/CSA James Webb Space Telescope to reveal, 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.

These new observations have found small, faint objects surrounding the 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 neighbouring galaxies is the source of this hydrogen emission. The results are reported in the journal Nature Astronomy.

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.

To study the 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 these galaxies, and probe the early history of the Universe.

The earliest galaxies were sites of vigorous and active star formation, and were rich sources of a type of light emitted by hydrogen atoms called Lyman-α emission. However, during the epoch of reionisation, an immense amount of neutral hydrogen gas surrounded these stellar nurseries. Furthermore, the space between galaxies was filled by more of this neutral gas than is the case today. The gas can effectively absorb and scatter this kind of hydrogen emission, so astronomers have long predicted that the abundant Lyman-α emission released in the early Universe should not be observable today.

This theory has not always stood up to scrutiny, however, as examples of 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?

“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,” said lead author Callum Witten from Cambridge’s Institute of Astronomy. “Many hypotheses have previously been suggested to explain the great escape of this ‘inexplicable’ emission.”

The team’s breakthrough came thanks to Webb’s 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.

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.

“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,” said co-author Sergio Martin-Alvarez from Stanford University.

The team then used computer simulations 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.

The team is planning follow-up observations with galaxies at various stages of merging, 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.

Callum Witten et al. ‘Deciphering Lyman-α emission deep into the epoch of reionization.’ Nature Astronomy (2024). DOI: 10.1038/s41550-023-02179-3

Adapted from an ESA press release.


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