Astronomers in Hawaii have analyzed the dimmest known star cluster in the sky and come away perplexed. The object, which orbits the Milky Way galaxy about 30,000 light-years away from Earth, is so mysterious it currently has two names: Ursa Major III / UNIONS 1 (UMa3/U1). It might be a normal cluster of stars we spotted at a unique moment, or more interestingly, it could be a dwarf galaxy rife with dark matter.
Dark matter is a theorized substance that explains the universe's missing mass. As astronomers peer out at the cosmos, we see only 5% of the total mass. The rest is hypothetically composed of dark matter and dark energy. We cannot directly observe these materials, but they do have gravitational effects on the observable universe. And UMa3/U1 is displaying some pretty unusual effects.
The group of stars was discovered with the Ultraviolet Near Infrared Optical Northern Survey (UNIONS) and Pan-STARRS observatories in Hawaii. Despite its nearness to the Milky Way, it escaped detection due to its strangely low luminosity. A team from the University of Victoria and Yale University followed up on the discovery using the W. M. Keck Observatory and Canada-France-Hawaii Telescope (CFHT), both of which are also in Hawaii. The image above shows the UMa3/U1 hiding in a deep sky image (left) and those same stars isolated from the background (right).
The new data, which was published in The Astrophysical Journal, proves UMa3/U1 is not just a random grouping of stars. Yale professor Marla Geha explains that data from Keck shows these stars are no strangers. "All the stars are moving through space at very similar velocities and appear to share similar chemistries," she said. That means they have probably been grouped together since they formed in the distant past.
Given its low mass, this star cluster should not be able to exist so near the Milky Way. That leaves two options. The less interesting one is that we've simply caught UMa3/U1 as it approached the galaxy but before it was torn apart. The other is that UMa3/U1 has more mass than it appears because it's heavy with dark matter. The data currently supports the latter conclusion, but more study is needed.
If UMa3/U1 turns out to be a tiny dark matter galaxy, that would support the prevailing Lambda Cold Dark Matter (LCDM) model of the Big Bang. It holds that large galaxies like the Milky Way attracted hundreds of small satellite galaxies as they formed, some of which were bound up in clouds of dark matter. If true, some of these dim star clusters should still be visible today, shielded from gravitational disruption by their dark matter envelopes.
The outcome of the investigation will determine which name UMa3/U1 gets to keep. Faint satellites of the Milky Way are usually named after the constellation in which they are discovered (Ursa Major III), but faint star clusters are named after the survey project that finds them (UNIONS). The team is hopeful that further study of UMa3/U1 will determine its true name and pave the way for a better understanding of the early universe.
