‘Messy’ galaxies in the early universe struggled to settle

(Press-News.org) Astronomers using the James Webb Space Telescope (JWST) have captured the most detailed look yet at how galaxies formed just a few hundred million years after the Big Bang – and found they were far more chaotic and messy than those we see today.

The team, led by researchers at the University of Cambridge, analysed more than 250 young galaxies that existed when the universe was between 800 million and 1.5 billion years old. By studying the movement of gas within these galaxies, the researchers discovered that most were turbulent, ‘clumpy’ systems that had not yet settled into smooth rotating disks like our own Milky Way.

Their findings, published in Monthly Notices of the Royal Astronomical Society, suggest that galaxies gradually became calmer and more ordered as the universe evolved. But in the early universe, star formation and gravitational instabilities stirred up so much turbulence that many galaxies struggled to settle.

“We don’t just see a few spectacular outliers – this is the first time we’ve been able to look at an entire population at once,” said first author Lola Danhaive from Cambridge’s Kavli Institute for Cosmology. “We found huge variation: some galaxies are beginning to settle into ordered rotation, but most are still chaotic, with gas puffed up and moving in all directions.”

The researchers used JWST’s NIRCam instrument in a rarely used ‘grism mode’ that captures faint light from ionised hydrogen gas in distant galaxies. Danhaive wrote new code to unravel the grism data, matching it with images from other JWST surveys to measure how gas was moving inside each galaxy.

“Previous results suggested massive, well-ordered disks forming very early on, which didn’t fit our models,” said co-author Dr Sandro Tacchella from the Kavli Institute and the Cavendish Laboratory. “But by looking at hundreds of galaxies with lower stellar masses instead of just one or two, we see the bigger picture, and it’s much more in line with theory. Early galaxies were more turbulent, less stable, and grew up through frequent mergers and bursts of star formation.”

“This work helps bridge the gap between the epoch of reionisation and the so-called cosmic noon, when star formation peaked,” said Danhaive, who is also affiliated with the Cavendish Laboratory. “It shows how the building blocks of galaxies gradually transitioned from chaotic clumps into ordered structures, and how galaxies such as the Milky Way formed.”

The results show how JWST allows scientists to probe galaxy dynamics at a scale that was impossible before. Future studies will aim to combine these findings with observations of cold gas and dust to paint a fuller picture of how the earliest galaxies took shape.

“This is just the beginning,” said Tacchella. “With more data, we’ll be able to track how these turbulent systems grew up and became the graceful spirals we see today.”

The research was supported in part by the Royal Society, the European Union, and the Science and Technology Facilities Council (STFC), part of UK Research and Innovation (UKRI). JWST is an international partnership between NASA, ESA and the Canadian Space Agency (CSA). The data for this result were captured as part of the JWST Advanced Deep Extragalactic Survey (JADES). Sandro Tacchella is a Fellow of St Edmund’s College, Cambridge. Lola Danhaive is a PhD student in the Centre for Doctoral Training (CDT) in Data Intensive Science.

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