An impossibly small clump of stars is orbiting the Milky Way without breaking apart, which could mean it is the least massive galaxy ever spotted. Although it only contains about 57 stars, it may be chock-full of dark matter.
The strange cluster, called Ursa Major III/UNIONS 1, orbits our galaxy about 33,000 light years from the solar system. It is the smallest and faintest聽satellite of the Milky Way ever found, by a significant margin.
at Yale University and his colleagues spotted it as part of a project called the Ultraviolet Near Infrared Optical Northern Survey, which uses several large telescopes in Hawaii to image huge swathes of the sky. They then performed follow-up observations using the Keck II telescope in Hawaii.
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They found that the stars in the cluster have a mass about 16 times the mass of the sun, corresponding to about 57 relatively聽small stars. 鈥淚t鈥檚 so low mass that it should have been torn apart by the Milky Way,鈥� he says. 鈥淔or that to have not been the case, either this thing has to be a galaxy, or it has to be a star cluster that we鈥檝e observed at a very special time right before its demise.鈥�
If the clump is in fact a galaxy, that means that it should be full of聽dark matter, which would create the gravitational pull necessary to hold the stars together.聽聽showed that if this dark matter is there, it should have a mass of around one billion times that of the sun to account for the velocities of the stars in the galaxy.
And if the object is a star cluster, it is unlikely to survive for more than about 400 million years 鈥� but the current observations are leaning towards it being a galaxy, says Cerny. 鈥淚t could be the very, very faintest and smallest galaxy we know of in the whole universe,鈥� he says. More precise measurements of the clustered stars, along with detections of other similar objects, should help clear it up.
The Astrophysical Journal
