Astronomers discover ancient star system orbiting around Milky Way using telescopes atop Mauna Kea

Astronomers have spotted from Hawaiʻi a tiny, ancient star system orbiting around our galaxy – the faintest, lowest-mass Milky Way satellite ever discovered and possibly one of the most dark matter-dominated systems known.

Named Ursa Major III/UNIONS 1, this system consists of just 60 stars that are over 10 billion years old, all bound by their own gravity and maybe even dark matter, according to a press release from the W.M. Keck Observatory.

The University of Victoria and Yale University led the study using three Hawaiʻi telescopes – W.M. Keck Observatory and Canada-France-Hawaiʻi Telescope on Mauna Kea, plus the University of Hawaiʻi Institute for Astronomy Pan-STARRS on Haleakalā.

The findings are published in a recent edition of The Astrophysical Journal.

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

UMa3/U1 was first detected using data obtained from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS) at CFHT and Pan-STARRS. The UNIONS project is an ambitious imaging survey of the northern sky in the optical and near-infrared conducted since 2017 by CFHT, Pan-STARRS, and Japan’s Subaru Telescope on
Mauna Kea.

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

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

“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 Yale University graduate student Will Cerny, the second author of the study.

How these stars have managed to stay a tight-knit group is remarkable, experts say. One possible explanation is that dark matter may be keeping them together.

“The object is so puny that its long-term survival is very surprising. 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,” says Cerny. “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.”

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

A companion study on UMa3/U1’s implications on the LCDM theory has been accepted for publication in The Astrophysical Journal and is available in preprint format on arXiv.org.

“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.

Conclusive evidence of the presence or lack of dark matter in UMa3/U1 is key 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. Ultra-faint Milky Way satellites are typically named after the constellation they are discovered in (in this case, Ursa Major) whereas ultra-faint star clusters are generally named after the survey project they were discovered in (UNIONS).

While this star system’s identity is still ambiguous, UMa3/U1 paves the way for new perspectives in cosmology.

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