JWST spots smallest galaxy outside our local universe

The giant El Gordo galaxy cluster has been used to spot a tiny galaxy beyond it NASA/ESA/J. Jee (Univ. of California, Davis)/J. Hughes (Rutgers Univ.)/F. Menanteau (Rutgers Univ. & Univ. of Illinois/Urbana-Champaign)/C. Sifon (Leiden Obs.)/R. Mandelbum (Carnegie Mellon Univ.)/L. Barrientos (Univ. Catolica de Chile)/K. Ng (Univ. of California, Davis)

The James Webb Space Telescope has glimpsed the smallest galaxy outside our local universe – and it is a thousand times less massive than the Milky Way.

The James Webb Space Telescope (JWST) has spotted the smallest galaxy outside our local universe by using the heaviest known cluster of galaxies, called El Gordo, as a giant lens.

El Gordo was first discovered in 2011. Follow-up measurements found it contained so much mass – the equivalent of 3 million billion suns – that it was at the very limit of what standard cosmological theory predicts. This huge mass makes it useful as a gravitational lens, bending and magnifying the light from stars and galaxies on the other side of it that would otherwise be invisible to us.

Now, Jose Diego at the University of Cantabria in Santander, Spain, and his colleagues have used JWST to observe the infrared light from El Gordo and found 28 new galaxies that hadn’t been visible before, as well as a dwarf galaxy that is just a billion times heavier than our sun. Even dwarf galaxies typically contain a few billion stars.

The team identified the dwarf galaxy from a strange fluctuation of light from another galaxy, called La Flaca, that El Gordo had stretched into a pancake-like arc. “The only way to explain it is there is a small, tiny galaxy right there, which is probably a dwarf galaxy,” says Diego.

The galaxy is believed to be a thousandth of the mass of the Milky Way. If this is confirmed, it would be the smallest galaxy observed outside our local universe – which includes the Milky Way and its associated galaxies. Its existence could present problems for some physics models, says Diego. “The fact that you can see this galaxy is inconsistent with some models of dark matter that predict that dark matter could have a high temperature.” Warm dark matter would have a velocity too high to form a galaxy this small, he says.

Diego’s team also spotted a red supergiant star, a type of star that hadn’t been seen outside our local universe, but that JWST could identify using its infrared sensors.

If stars that fluctuate in brightness – which also tend to be red like this supergiant – are found by JWST, then they could be used to accurately estimate distances for very distant objects and measure the expansion of the universe. Currently, we use supernovae for this, which can be inaccurate.

Seeing such distant dwarf galaxies and red supergiant stars for the first time is “pretty amazing”, says Felipe Menanteau at the University of Illinois at Urbana-Champaign. However, on their own, these objects won’t be enough to help inform our models of the universe’s formation; more data points will be needed for that, he says.


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