Astronomers find a tiny star system with a tantalizing story to tell

Fri, 04/05/2024

Astronomers find a tiny star system with a tantalizing story to tell

If confirmed as a galaxy, the system would be the faintest galaxy ever discovered — and may suggest that many others remain to be discovered. 

Yale astronomers have helped identify a tiny star system orbiting the Milky Way which they say hints at the existence of a new class of faint, satellite star systems that orbit around large galaxies.

An international team led by researchers at Yale and the University of Victoria in Canada recently announced the discovery of Ursa Major III/UNIONS 1 (UMa3/U1), the faintest and lowest-mass star system ever found.

The newly discovered system is only 20 light years across (more than 58.7 trillion miles) and contains only about 60 “mature” stars — mature in this case meaning more than 10 billion years old. Its mass is 15 times less than the mass of the dimmest dwarf galaxy.

The researchers discovered UMa3/U1 and studied it in detail using the W.M Keck Observatory’s Deep Imaging Multi-Object Spectrograph (DEIMOS) in Hawai’i, the Ultraviolet Near Infrared Optical Northern Survey (UNIONS) at the Canada-France-Hawai’i Telescope, and Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) at the University of Hawai’i Institute for Astronomy.

There are so few stars in Uma3/U1 that one might reasonably question whether it’s just a chance grouping of similar stars,” said Marla Geha, a professor of astronomy and physics in Yale’s Faculty of Arts and Sciences and co-author of a new study published in The Astrophysical Journal.

Keck was critical in showing this is not the case,” Geha said. “Our DEIMOS measurements clearly show all the stars are moving through space at very similar velocities and appear to share similar chemistries.”

For now, the new star system has two names, due to the fact that astronomers can’t yet confirm whether it is a dwarf galaxy (named for its constellation, Ursa Major) or a star cluster (named after the survey in which it was discovered, UNIONS).

At the heart of this question is the possible involvement of dark matter — unseen matter that is thought to be the invisible “scaffolding” of the universe.

Excitingly, a tentative spread in velocities among the stars in the system may support the conclusion that UMa3/U1 is a dark matter dominated galaxy, a tantalizing possibility we hope to scrutinize with more Keck observations,” said William Cerny, a Yale graduate student in Geha’s research group and second author of the new study.

The object is so puny that its long-term survival is very surprising,” Cerny added. “One might have expected the harsh tidal forces from the Milky Way’s disk to have ripped the system apart by now, leaving no observable remnant. The fact that the system appears intact leads to two equally interesting possibilities. Either UMa3/U1 is a tiny galaxy stabilized by large amounts of dark matter, or it’s a star cluster we’ve observed at a very special time before its imminent demise.”

The answer may have ramifications well beyond the star system’s eventual name.

The standard cosmological model of the universe, known as the Lambda Cold Dark Matter model, predicts that when galaxies such as the Milky Way formed, they exerted enough gravitational pull to attract hundreds of small satellite star systems down to very small masses, which continue to orbit the larger galaxies.

If UMa3/U1 is a galaxy, its existence implies that many such faint satellite galaxies remain to be discovered.

Whether future observations confirm or reject that this system contains a large amount of dark matter, we’re very excited by the possibility that this object could be the tip of the iceberg — that it could be the first example of a new class of extremely faint stellar systems that have eluded detection until now,” Cerny said.

Simon Smith, a graduate student at the University of Victoria, is the study’s first author.

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