Galaxy smash-ups may explain strange light from early universe

Observations from the James Webb Space Telescope (JWST) have revealed dim galaxies smashing together, which could solve the mystery of previously unexplained glints of light from early in the history of the cosmos.

For a long period of the universe’s history ending about 1 billion years after the big bang, space was full of a pristine gas that should have blocked out the copious light emitted by hydrogen atoms. But researchers have seen twinkles of hydrogen shining from many galaxies in the early universe. This is a type of light known as Lyman-alpha emission.

How this light escaped the shroud of gas has baffled astronomers, but Callum Witten at the University of Cambridge and his colleagues have found a potential solution. They examined JWST images of nine distant galaxies, all putting out Lyman-alpha emission, and found that every single one had at least one smaller galaxy right next to it. These secondary galaxies were too faint to be spotted with previous telescopes, and they all appear to be merging with their brighter partners.

Merging galaxies create bursts of star formation and light, including Lyman-alpha emission. They also generate powerful winds that could blow away the galaxies’ cosmic gas, allowing the light to escape. Those winds and the energy from the star formation could also strip the gas atoms of their electrons, which would otherwise allow it to absorb the light, rendering it transparent.

“We were aware there was a chance that we were missing fainter galaxies, but we weren’t aware that there would be so many so close to these brighter galaxies,” says Witten. “We weren’t aware that they were helping allow this emission to get out.”

The researchers ran a series of simulations to test their hypothesis, and they found that the interactions between the galaxies did indeed create odd channels through the gas, allowing the hydrogen emission to leak out in such a way that our telescopes could spot it. “We had a sort of biased view of these very early galaxies before, and it failed to account for the chaotic process of them merging,” says Witten. “This emission we thought shouldn’t exist, this has explained that.”

There are other possible explanations as well, including turbulence from active black holes at the centres of these galaxies, but it seems that galactic mergers must play a significant role, says Witten. However, with a sample of only nine galaxies, we cannot be sure it is the only answer.

Witten and his colleagues are waiting for JWST data on more Lyman-alpha emitters to become publicly available, and while they do so they are looking at other merging galaxies to understand the process more precisely. “To really prove this hypothesis, we’ll have to see how this holds up when we detect dozens more, if not a few hundred, in the coming years,” says Aayush Saxena at the University of Oxford, who was not involved in this work. “If we continue to find these merging galaxies, then that mystery will really be solved.”