Ursa Major III/UNIONS 1 (UMa3/U1 for short) is the least luminous and lowest-mass Milky Way satellite ever discovered, and possibly one of the most dark matter-dominated systems known.
Hidden in this deep sky image (left) is Uma3/U1, a miniscule group of stars (right) bound together by their own gravity in orbit around the Milky Way Galaxy. Image credit: CFHT / S. Gwyn / S. Smith.
UMa3/U1 has a mass of only 16 solar masses and consists of only five dozen bright stars spread over a volume just 20 light-years across.
It was likely born long ago in the early days of the Universe, as it consists of stars that appear to be more than 10 billion years old.
“UMa3/U1 is located in the constellation of Ursa Major,” said Simon Smith, a Ph.D. student at the University of Victoria and first author of a paper published in the Astrophysical Journal.
“It is in our cosmic backyard, relatively speaking, at about 30,000 light-years from the Sun.”
“UMa3/U1 had escaped detection until now due to its extremely low luminosity.”
“It is either the faintest ancient star cluster known to date, or the faintest and closest known dwarf galaxy ever discovered.”
“Being able to detect such a tiny system, with only about 60 stars, speaks for the quality of the data that the team had to work with.”
UMa3/U1 was first detected using data obtained from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS) at CFHT and Pan-STARRS.
Smith and his colleagues then studied the star system in finer detail using Keck Observatory’s Deep Imaging Multi-Object Spectrograph (DEIMOS).
“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 Yale University’s Professor Marla Geha.
“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.
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, astronomers 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 which continue to orbit galaxies today.
A companion study on UMa3/U1’s implications on the LCDM theory will be published in the Astrophysical Journal.
“The presence of dark matter — the key distinguishing property between dwarf galaxies and star clusters — is highly likely, because the satellite’s orbit takes it through the inner regions of the Milky Way, where gravitational tidal forces are strongest,” Smith said.
“Without the binding presence of large amounts of dark matter, the satellite would not have survived long on its current orbit; it would have been pulled apart by those forces.”
“This is a momentous discovery, and fully consistent with a long-standing prediction of the LCDM theory,” said Dr. Julio Navarro, an astronomer at the University of Victoria.
“This may be the faintest galaxy known, and also one of the most dark matter-dominated. It’s hard to see what else its unique set of properties might mean.”
“Estimating the dark matter content of a dwarf galaxy requires accurate and repeated measurements of its stellar velocities,” said Dr. Raphaël Errani, an astronomer at the Carnegie Mellon University.
“Remarkably, the measurements obtained with the Keck II telescope are tentatively consistent with those predicted by the LCDM theory.”
“Without dark matter it is not obvious how UMa3/U1 could have been able to survive unscathed for billions of years.”
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Simon E.T. Smith et al. 2024. The Discovery of the Faintest Known Milky Way Satellite Using UNIONS. ApJ 961, 92; doi: 10.3847/1538-4357/ad0d9f
Raphaël Errani et al. 2024. Ursa Major III/UNIONS 1: the darkest galaxy ever discovered? ApJ, in press; arXiv: 2311.10134
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