April 13, 2024

Faintest known star system orbiting the Milky Way discovered

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Hidden in this deep-sky image (left) is Uma3/U1, a tiny group of stars (right) held together by their own gravity (and possibly even dark matter!) in orbit around the Milky Way. Credit: CFHT/S. Gwyn (right) / S. Smith (left). The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad0d9f

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Hidden in this deep-sky image (left) is Uma3/U1, a tiny group of stars (right) held together by their own gravity (and possibly even dark matter!) in orbit around the Milky Way. Credit: CFHT/S. Gwyn (right) / S. Smith (left). The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad0d9f

A team of astronomers led by the University of Victoria and Yale University has detected an ancient star system traveling around our galaxy called Ursa Major III/UNIONS 1 (UMa3/U1) – the faintest and lowest-mass Milky Way satellite ever. discovered, and possibly one of the most dark matter-dominated systems known.

The team conducted the study in Hawaii using two Maunakea Observatories on Hawaii Island – WM Keck Observatory and Canada-France-Hawaii Telescope (CFHT) – as well as the University of Hawaii’s Pan-STARRS Institute for Astronomy (Panoramic Survey Telescope and System Rapid Response Center) in Haleakalā, Maui; The findings were published in a recent issue of The Astrophysical Journal.

“UMa3/U1 is located in the constellation Ursa Major (Ursa Major), home to the Big Dipper. It’s in our cosmic backyard, relatively speaking, about 30,000 light-years from the Sun,” says astronomy graduate Simon Smith. student at the University of Victoria and lead author of the study. “UMa3/U1 has escaped detection until now due to its extremely low luminosity.”

Observations reveal that the star system is tiny, with only about 60 stars over 10 billion years old, spanning just 10 light-years in diameter. UMa3/U1 has an extremely low mass – at 16 times the mass of the Sun, it is 15 times less massive than the faintest suspected dwarf galaxy.

UMa3/U1 was first detected using data obtained from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS) at CFHT and Pan-STARRS.

The team then studied the star system in more detail using the Keck Observatory’s Deep Imaging Multi-Object Spectrograph (DEIMOS) and confirmed that UMa3/U1 is a gravitationally bound system, whether a dwarf galaxy or a star cluster.

“There are so few stars in UMa3/U1 that one could reasonably question whether it is just a random grouping of similar stars. Keck was critical in showing that this is not the case,” says co-author Marla Geha, professor of astronomy and physics at the University of Michigan. Yale University. “Our DEIMOS measurements clearly show that all stars move through space at very similar speeds and appear to share similar chemistries.”

“It’s exciting that an attempt to propagate velocities among the system’s stars could support the conclusion that UMa3/U1 is a galaxy dominated by dark matter – a tantalizing possibility that we hope to examine with more Keck observations,” says Will Cerny, graduate student graduate program at Yale University. , the second author of the study.

It’s remarkable how these stars have managed to remain a united group. One possible explanation is that dark matter may hold them together.

“The object is so insignificant that its long-term survival is very surprising. One would expect that the strong tidal forces of the Milky Way’s disk would have already destroyed the system, leaving no observable trace,” says Cerny. “The fact that the system appears intact leads to two equally interesting possibilities. Either UMa3/U1 is a small galaxy stabilized by large amounts of dark matter, or it is a star cluster that we observed at a very special moment, before its imminent disappearance.”

With the first scenario, getting direct confirmation of UMa3/U1 as a faint, ancient, dark matter-dominated satellite star system would be an exciting feat because it would support a prediction in the leading theory for the origin of the universe.

Under the Lambda Cold Dark Matter (LCDM) model, scientists hypothesize that when galaxies like the Milky Way formed, they created a gravitational pull during their assembly process that attracted hundreds of satellite star systems that continue to orbit galaxies today. .

A complementary study on the implications of UMa3/U1 in LCDM theory has been accepted for publication in The Astrophysical Journal and is available in pre-printed format on the website arXiv server.

“Whether future observations confirm or reject that this system contains a large amount of dark matter, we are very excited about 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 complex objects. faint stellars that have escaped detection until now,” says Cerny.

Conclusive evidence for the presence or lack of dark matter in UMa3/U1 is critical to determining whether the star system is a dwarf galaxy or a star cluster. Until its classification becomes clear, Ursa Major III/UNIONS 1 has two names. The Milky Way’s ultrafaint satellites are typically named after the constellation in which they are discovered (in this case, Ursa Major), while ultrafaint star clusters are usually named after the research project in which they were discovered (UNIONS).

Although the identity of this star system is still ambiguous, UMa3/U1 opens the way to new perspectives in cosmology.

“This discovery could challenge our understanding of galaxy formation and perhaps even the definition of a ‘galaxy’,” says Smith.

More information:
Simon ET Smith et al, The discovery of the faintest known Milky Way satellite using UNIONS, The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad0d9f Raphaël

Errani et al, Ursa Major III/UNIONS 1: the darkest galaxy ever discovered?, arXiv (2023). DOI: 10.48550/arxiv.2311.10134

Diary information:
Astrophysical Journal

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