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.
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…
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