Signals give astronomers ‘unexpected' insight into early galaxy formation

The Guardian

26-03-2025

Astronomers have detected signals from a momentous event in the early universe in which a dense fog that cloaked the first stars began to lift, marking the end of the cosmic dark ages.
Until now, the exact timing and nature of this critical transition has remained shrouded in mystery. The latest images, from the James Webb space telescope, reveal a galaxy dated to just 330m years after the big bang that appears to have cleared its surrounding fog.
'We could think of the early universe as shrouded with a thick fog that would make it exceedingly difficult to find even powerful lighthouses peeking through, yet here we see the beam of light from this galaxy piercing the veil,' said Dr Kevin Hainline, an astronomer at the University of Arizona and co-author of the findings.
The observations are considered significant because they put a very early timestamp on the transition that saw the universe become transparent, allowing high energy starlight to travel freely across space for the first time.
'It is one of the key moments in the lifetime of the universe,' said Prof Roberto Maiolino, an astrophysicist at the University of Cambridge and a team member.
Shortly after the big bang, the universe was filled with neutral hydrogen gas that absorbed almost all ultraviolet light emitted by the first stars (visible light would have escaped). As the earliest galaxies grew in size and radiated more powerful energy, they began to burn away the fog in their local area by splitting (ionising) hydrogen atoms into free floating protons and electrons that were no longer able to efficiently absorb light.
For the first time, high energy starlight could travel freely, initially through bubbles of plasma surrounding the largest galaxies and eventually, as these bubbles grew and merged, across the entire cosmos. Astronomers agree that this process, known as the epoch of reionization, was complete by approximately 1bn years after the big bang.
The latest observations reveal a clear signature of UV light being emitted from the GS-z13-1 galaxy. This suggests the galaxy must have been surrounded by a plasma bubble at least 650,000 light years across, the astronomers estimate. Beyond that distance, UV light would have been red-shifted into the visible light range that is less likely to be captured by hydrogen gas.
'GS-z13-1 is seen when the universe was only 330m years old, yet it shows a surprisingly clear, telltale signature … that can only be seen once the surrounding fog has fully lifted,' said Maiolino. 'This result was totally unexpected by theories of early galaxy formation and has caught astronomers by surprise.'
The strength of the signal suggests that the early galaxy may have contained gigantic stars, up to 300 times the mass of our own sun, and 15 times as hot. 'We're seeing such a strong feature that the intrinsic strength of the [radiation] should be enormous,' said Joris Witstok of the University of Copenhagen and first author. 'The large bubble of ionised hydrogen surrounding this galaxy might have been created by a peculiar population of stars – much more massive, hotter and more luminous than stars formed at later epochs, and possibly representative of the first generation of stars.'
Dr Emma Chapman, an astrophysicist at the University of Nottingham who was not involved in the observations, said: 'This James Webb space telescope observation of a single, faint galaxy living through the epoch of reionization is a defining moment of the search, like finding the first buried arrowhead on an ancient battlefield. We need many more observations to truly understand how the battle for the ionization of the universe progressed, but we can now go on digging knowing we are in the right place and time.'
The findings are published in the journal Nature.