A collection of stars 30,000 light-years away is the faintest and lowest-mass Milky Way satellite ever found, according to the group of scientists who recently observed it. Oh, and it may be dominated by dark matter, the unknown stuff that makes up about 27% of the universe.
It’s a big surprise that this system, sitting on the edge of our galaxy and subject to the gravitational force of its disk, has managed to persist. A team of researchers that studied the stellar grouping concluded that the stars have stayed together because they are either a dwarf galaxy or a star cluster, gravitationally bound together.
The team published its analysis earlier this year in The Astrophysical Journal, and a paper discussing the implications for the system as it pertains to the Lambda Cold Dark Matter (LCDM) model, a leading model hypothesizing the origins of the cosmos, is currently hosted on the preprint server arXiv.
The team used the Keck Observatory’s Deep Imaging Multi-Object Spectrograph to confirm that the stars were gravitationally tied together. “There are so few stars in UMa3/U1 that one might reasonably question whether it’s just a chance grouping of similar stars. Keck was critical in showing this is not the case,” said study co-author Marla Geha, an astrophysicist at Yale University, in a Keck release. “Our DEIMOS measurements clearly show all the stars are moving through space at very similar velocities and appear to share similar chemistries.”
The satellite is called Ursa Major III / UNIONS 1 (UMa3/U1 for short), and is so named for the constellation it’s in and the survey project that first revealed it. Regardless of its true identity, it’s minuscule, constituted by around 60 10-billion-year-old stars in a region of space about 10 light-years wide. The mass of the entire system is just 16 times that of the Sun. If it is a dwarf galaxy, it’s 15 times smaller than the second-faintest dwarf galaxy that’s known to astronomers.
“UMa3/U1 had escaped detection until now due to its extremely low luminosity,” said Simon Smith, a researcher at the University of Victoria and lead author of the new paper, in the same release. “This discovery may challenge our understanding of galaxy formation and perhaps even the definition of a ‘galaxy.’”
The team concluded that UMa3/U1 may be dominated by dark matter based on the spread of velocities of stars in the system. The dark matter acts as a gravitational glue, holding the stars in their group. While scientists do not know what dark matter is, they observe its gravitational effects on visible matter. Dark matter is generally thought to be a hitherto unknown particle or set of particles, such as axions, though other objects—like primordial black holes from the very beginning of the universe—are also in the conversation.
If dark matter isn’t responsible for the system—and follow-up observations by the Keck Observatory may answer that question—its a group of stars seen at the very end of their lives.