Astronomers have discovered what may be the faintest dwarf galaxy in the Milky Way. This galaxy contains only 60 stars and could be the remnant of a distant galaxy from the Milky Way, or even a globular cluster. A preprint of this study is available at arXiv.org. Dwarf galaxies are interesting for several reasons. First, its dynamics cannot be explained solely by a combination of baryonic matter processes and Newton’s laws, and the lambda CDM cosmological model predicts that dwarf galaxies are surrounded by a halo of dark matter. Therefore, such galaxies can be used to constrain dark matter models. Second, the star formation and chemical enrichment history of such galaxies may be different from that of large galaxies.
Dwarf galaxies, which may be satellites of the Milky Way, lie within its virial radius and are very faint. They are difficult to find, not only because of their small size and brightness, but also because they need to be distinguished from globular clusters. A team of astronomers led by Simon E.T. Smith of the University of Victoria reports the discovery of the Milky Way’s faintest dwarf galaxy candidate, Ursa Major III/UNIONS 1 (UMa3/U1), as part of the Local Data Group’s UNIONS deep exploration program Did. Wide-area survey of faint galaxies. It includes ultraviolet, near-infrared, and optical data from his four telescopes on the ground. The discovery was confirmed using observational data from the Keck Observatory and the Gaia Space Telescope.
The galaxy’s absolute brightness in the visible range is +2.2. UMa3/U1 stars have masses between 11 and 22 solar masses and contain only 50 to 60 old (more than 11 billion years), metal-poor ([Fe/H] ~ -2.2) stars. The mass of the stars in this galaxy is only a quarter of the mass of the Milky Way’s faintest moon to date. It is located 32.61 million light-years from the Sun and has an effective radius of only 10 light-years.
UMa3/U1’s orbit passes through the disk of the Milky Way galaxy, about 52.1 thousand light-years from its center. Repeated interactions with the Milky Way’s outer disk may have led to the tidal separation of some stars and the accumulation of galactic material in our galaxy. The similarities in the orbits of UMa3/U1 and globular cluster M68 are also interesting. If UMa3/U1 is the remnant of a dwarf galaxy destroyed by tidal action, it may have contained M68 in the past. If UMa3/U1 is a star cluster, UMa3/U1 and M68 may have formed in the same environment. So far, the nature of UMa3/U1 and the proportion of dark matter in it are not fully understood. Its status should be determined by future observations.