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A new group of stars has just been found, and its nature intrigues scientists: it could be both an ancient star cluster and the dwarf galaxy with the largest amount of dark matter ever seen. Whatever its nature, the object can certainly help scientists better understand the formation of the Milky Way.
The star system is about 30 thousand light-years from Earth, and was called Ursa Major III/Unions 1 (UMa3/U1) because it is in the constellation Ursa Major. It has only 60 visible stars in an area 10 light years wide, and together the stars are only 16 times the mass of the Sun. This is where the biggest mystery lies.
“The fact that this system appears to be intact leads us to two equally interesting possibilities,” said study co-author Will Cerny. “Either UMa3/U1 is a tiny galaxy stabilized by large amounts of dark matter, or it is a cluster of stars that we observed at a very special moment before its imminent death,” he suggested.
If UMa3/U1 is a dwarf galaxy, it could help astronomers answer some questions about the formation of the Milky Way. The biggest issue here is that, according to the standard model of cosmology, the formation of galaxies is a hierarchical process. This means that dark matter halos with dwarf galaxies merge and form larger galaxies.
As this process is continuous, it is expected that there will still be some dwarf galaxies traveling through space. However, only a few dozen of them were found by scientists, who wonder where the rest are. Some of them are the so-called ultra-faded dwarf galaxies (or UFDs), and they house a few thousand stars in a halo of dark matter.
But in the case of UMa3/U1, its mass is 15 times smaller than the least massive known UDF. If this galaxy is also a dwarf, then those that are lost could, in fact, have been where scientists expected all along, and would have gone unnoticed because they have few stars.
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Simon Smith, the author who led the study, suspects that UMa3/U1 must actually be a dwarf galaxy, not a star cluster. The conclusion comes from data from the WM Keck observatory in Hawaii. He and the other authors measured the velocities of the star system and noticed something interesting.
In the image below, you can see the part of the sky where UMa3/U1 is (left) and its isolated stars (right):
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The dispersion speed (that between the fastest and slowest stars) indicates that they maintain their positions thanks to a halo of dark matter. If this is the case, then UMa3/U1 has one of the highest proportions ever seen of dark matter relative to baryonic, visible matter.
One of the articles with the results of the study was published in the journal The Astrophysical Journal. The other was approved for publication, and can be accessed in the online repository arXiv.
Source: The Astrophysical Journal, arXiv; Via: Carnegie Mellon University
