How do you distinguish a galaxy from a mere cluster of stars? That’s easy, right? A galaxy is a large collection of millions or billions of stars, while a star cluster only has a thousand or so. Well, that kind of thinking won’t get you a Ph.D. in astronomy! Seriously, though, the line between galaxy and star cluster isn’t always clear. Case in point, UMa3/U1.
It’s easy to distinguish galaxies such as Andromeda and the Milky Way. They are large, gravitationally bound, and dominated by dark matter. It’s also easy to distinguish star clusters such as the Pleiades. They are loosely bound star groupings without dark matter. But for a type of small dwarf galaxy known as Ultra-Faint Dwarfs (UFDs) the dividing line gets fuzzy.
UFDs are dominated by dark matter. The mass of the Milky Way, for example, is about 85% dark matter. An ultrafaint dwarf galaxy, however, can have a thousand times more dark matter than luminous matter. This is why they are so faint. Since UFDs often contain some of the oldest stars in the Universe, astronomers love to study them for clues on the origins of galaxies. Which brings us to UMa3/U1.
Even its name tells us that there is a problem. If, in fact, the object is a dwarf galaxy then its name should be Ursa Major III, as it is a satellite galaxy in the constellation Ursa Major. If it is an ancient star cluster, then it should be named UNIONS 1, since it was discovered by the Ultraviolet Near Infrared Optical Northern Survey (UNIONS). If it is a galaxy, then it is the smallest and most dark-matter dominated galaxy yet discovered. If it is a star cluster, then with an age of about 11 billion years, it is the oldest star cluster yet discovered.
UMa3/U1 is downright tiny. It is only 20 light-years across, contains only about 60 stars, and has a visible mass of just 16 Suns. In comparison, the Pleiades has about the same diameter, but contains more than 1,000 stars and 800 solar masses. So the real question for UMa3/U1 is whether it is dominated by dark matter.
In a recent study, the team looked at several tests to distinguish star clusters and dwarf galaxies. Their first approach was to look at the dynamics of the visible stars, assuming it is a star cluster. Based on their known motions, the team simulated how long it would take for the stars to break free, a process known as evaporation. Based on their simulations, the cluster could survive for another 2-3 billion years. That’s a good fraction of the estimated 11 billion year age, which suggests U1 is simply a stable star cluster.
The second test the team applies is what is known as the mass function. This is a plot of the way the mass of the cluster varies with distance. If it is a cluster, then the mass should be more evenly distributed, but if it is a galaxy, stars should be clustered toward the center. Here the data is less conclusive. The distribution of visible stars is a decent match to the cluster model, but for a galaxy the central stars would be mostly white dwarfs and neutron stars, which are too dim to distinguish with current observations.
Overall, the evidence leans towards UMa3/U1 being a star cluster, but the team notes that it will take more observations of other UFDs to be conclusive. Fortunately, upcoming telescopes such as the Vera Rubin Observatory will discover many more faint dwarfs in time.
The original version of this article was published on Universe Today.
