Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.
The Discovery of the Faintest Known Milky Way Satellite Using UNIONS
Published January 2024
Main takeaway:
A team led by Simon Smith (University of Victoria) discovered a collection of stars orbiting the Milky Way in data from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), which is carried out by three telescopes in Hawaiʻi. The newly discovered Milky Way satellite, named Ursa Major III/UNIONS 1, contains about 57 stars and has a total mass of 16 solar masses. Ursa Major III/UNIONS 1 is the faintest known Milky Way satellite.
Why it’s interesting:
The advent of sensitive surveys has revealed much about the Milky Way’s neighborhood, including a steadily growing population of satellites. These surveys are discovering fainter and fainter satellites that blur the line between the largest star clusters and the smallest dwarf galaxies. Dwarf galaxies and star clusters have significant overlap in their masses and are distinguished by the presence or absence of dark matter: dwarf galaxies are thought to form in individual dark matter halos, while star clusters are not.
The brightness and size of Ursa Major III/UNIONS 1 (orange square) relative to dwarf galaxies (blue circles), globular clusters (red circles), and ambiguous satellites (black diamonds). Click to enlarge. [Smith et al. 2024]
On the nature of this faint satellite:
It’s not yet clear whether Ursa Major III/UNIONS 1 is a dwarf galaxy or a star cluster. This fact is reflected in the system’s name: newfound dwarf galaxies are named for the constellation in which they appear, while star clusters are named for the survey in which they were discovered. This satellite currently bears both kinds of names. Smith’s team found evidence that the velocities of stars in Ursa Major III/UNIONS 1 are spread fairly widely around the mean velocity of the system, suggesting that the system sits in its own dark matter halo and thus could be an astoundingly small dwarf galaxy. However, this finding is highly sensitive to the number of stars included in the analysis, highlighting the need for careful follow-up observations of this faint Milky Way satellite — and many others.
Citation
Simon E. T. Smith et al 2024 ApJ 961 92. doi:10.3847/1538-4357/ad0d9f