Drawing the Line | by Brian Koberlein

CFHT / S. Gwyn / S. Smith

How do you distinguish a galaxy from a mere cluster of stars? That’s straightforward, proper? A galaxy is a big assortment of tens of millions or billion of stars, whereas a star cluster solely has a thousand or so. Effectively, that form of considering received’t get you a Ph.D. in astronomy! Significantly, although, the road between galaxy and star cluster isn’t all the time clear. Living proof, UMa3/U1.

It’s straightforward to differentiate galaxies corresponding to Andromeda and the Milky Approach. They’re giant, gravitationally sure, and dominated by darkish matter. It’s additionally straightforward to differentiate star clusters such because the Pleiades. They’re loosely sure star groupings with out darkish matter. However for a kind of small dwarf galaxy generally known as Extremely-Faint Dwarfs (UFDs) the dividing line will get fuzzy.

UFDs are dominated by darkish matter. The mass of the Milky Approach, for instance, is about 85% darkish matter. An ultrafaint dwarf galaxy, nonetheless, can have a thousand instances extra darkish matter than luminous matter. Because of this they’re so faint. Since UFDs usually comprise a number of the oldest stars within the Universe, astronomers love to review them for clues on the origins of galaxies. Which brings us to UMa3/U1.

Even its title tells us that there’s a drawback. If, in actual fact, the item is a dwarf galaxy then its title ought to be Ursa Main III, as it’s a satellite tv for pc galaxy within the constellation Ursa Main. Whether it is an historical star cluster, then it ought to be named UNIONS 1, because it was found by the Ultraviolet Close to Infrared Optical Northern Survey (UNIONS). If it’s a galaxy, then it’s the smallest and most dark-matter dominated galaxy but found. If it’s a star cluster, then with an age of about 11 billion years, it’s the oldest star cluster but found.

UMa3/U1 is downright tiny. It is just 20 light-years throughout, comprises solely about 60 stars, and has a visual mass of simply 16 Suns. As compared, the Pleiades has about the identical diameter, however comprises greater than 1,000 stars and 800 photo voltaic lots. So the true query for UMa3/U1 is whether or not it’s dominated by darkish matter.

In a latest research, the workforce checked out a number of exams to differentiate star clusters and dwarf galaxies. Their first method was to have a look at the dynamics of the seen stars, assuming it’s a star cluster. Based mostly on their identified motions, the workforce simulated how lengthy it will take for the celebs to interrupt free, a course of generally known as evaporation. Based mostly on their simulations, the cluster may survive for an additional 2-3 billion years. That’s an excellent fraction of the estimated 11 billion yr age, which suggests U1 is just a steady star cluster.

The second check the workforce applies is what is called the mass perform. It is a plot of the way in which the mass of the cluster varies with distance. If it’s a cluster, then the mass ought to be extra evenly distributed, but when it’s a galaxy, stars ought to be clustered towards the middle. Right here the info is much less conclusive. The distribution of seen stars is an honest match to the cluster mannequin, however for a galaxy the central stars could be principally white dwarfs and neutron stars, that are too dim to differentiate with present observations.

Total, the proof leans in the direction of UMa3/U1 being a star cluster, however the workforce notes that it’ll take extra observations of different UFDs to be conclusive. Thankfully, upcoming telescopes such because the Vera Rubin Observatory will uncover many extra faint dwarfs in time.