The Milky Way’s Smallest, Faintest Satellite Galaxy Found

The Milky Way has many satellite galaxies, most notably the Large and Small Magellanic Clouds. They’re both visible to the naked eye from the southern hemisphere. Now astronomers have discovered another satellite that’s the smallest and dimmest one ever detected. It may also be one of the most dark matter-dominated galaxies ever found.

The galaxy is called Ursa Major III / UNIONS 1 (UMa3/U1), and it contains very few stars. In fact, its luminosity is so low that it’s gone undetected until new, even though it’s in our neighbourhood.

The discovery is in a new paper titled “Ursa Major III/UNIONS 1: the darkest galaxy ever discovered?” The paper has been published in The Astrophysical Journal, and the lead author is Simon Smith. Smith is an astronomy graduate student at the University of Victoria, BC, Canada.

“UMa3/U1 is located in the Ursa Major (Great Bear) constellation, home of the Big Dipper. It is in our cosmic backyard, relatively speaking, at about 30,000 light-years from the Sun,” said Smith. “UMa3/U1 had escaped detection until now due to its extremely low luminosity.”

There are only about 60 stars in UMa3/U1, which barely qualifies it as a galaxy. There are star clusters with more members than that. In fact, the tiny galaxy is more in line with an open cluster in terms of number of stars.

“There are so few stars in UMa3/U1 that one might reasonably question whether it’s just a chance grouping of similar stars.”

Marla Geha, professor of astronomy and physics at Yale University

The tiny galaxy contains stars that are more than 10 billion years old and is only 10 light-years across, small for a galaxy. Its mass is also low for a galaxy. It contains just 16 times the mass of the Sun and is 15 times less massive than the faintest suspected dwarf galaxy. Those are small numbers more similar to a globular cluster, but it still might be a galaxy because of the presence of dark matter.

While stellar associations like globular clusters are more massive than this dwarf galaxy, they’re not galaxies. Astronomers think that globulars are dominated by baryonic (normal) matter processes. Ultra-faint galaxies (UFG) like this one have masses many orders of magnitude greater than their stars can account for. “Therefore, in the framework of ?CDM (Lambda Cold Dark Matter) cosmology, dwarf galaxies are thought to lie at the center of their own dark matter halos,” the research states. Astrophysicists think the dark matter haloes account for all that mass, something that globulars and other star clusters lack.

The tiny galaxy was first spotted as part of the Ultraviolet Near Infrared Optical Northern Survey (UNIONS) at Canada France Hawaii Telescope (CFHT) and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS,) both in Hawaii. Once detected, the researchers studied it in more detail with Keck Observatory’s Deep Imaging Multi-Object Spectrograph (DEIMOS). Those observations confirmed that the stars are gravitationally bound, meaning they had to be either in a cluster or a tiny galaxy.

The CFHT is at Hawaii’s Mauna Kea Observatory, Hawaii, and Pan-STARRS is at the Haleakala Observatory in Hawaii. Both are key parts of UNIONS, the Ultraviolet Near Infrared Optical Northern Survey. Image Credit: UNIONS

The galaxy’s small number of stars would make anyone question whether it can be rightly called a galaxy. Even the researchers had their doubts.

“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,” says co-author Marla Geha, professor of astronomy and physics at Yale University. “Our DEIMOS measurements clearly show all the stars are moving through space at very similar velocities and appear to share similar chemistries.”

This figure from the research shows the motion (L) and velocity (R) of the dwarf galaxy’s member stars. In the left panel, the great region marks the motion of stars in the Milky Way and shows how the member stars (blue) are clustered together differently. In the panel on the right, the member stars are clustered together by velocity, and the empty circles are other non-member stars. Image Credit: Smith et al. 2024

Astronomers have struggled to understand dwarf galaxies and their dark matter. For one thing, the diagnostics astronomers use, like the stellar mass-metallicity relation, leads to arguments that they’re more like star clusters than galaxies. Also, their observed properties place them at the mid-point between clusters and dwarf galaxies.

Uncertainty abounds when it comes to UMa3/U1. Somehow, this association of stars has remained intact for a long time. With such low stellar mass, the grouping should’ve been torn apart by now, its members diluted into the larger Milky Way population. The fact that it’s still together is an intriguing indication that dark matter is involved.

“The object is so puny that its long-term survival is very surprising.”

Will Cerny, co-author, Yale University

“Excitingly, a tentative spread in velocities among the stars in the system may support the conclusion that UMa3/U1 is a dark matter-dominated galaxy – a tantalizing possibility we hope to scrutinize with more Keck observations,” said Yale University graduate student Will Cerny, the second author of the study.

“The object is so puny that its long-term survival is very surprising. One might have expected the harsh tidal forces from the Milky Way’s disk to have ripped the system apart by now, leaving no observable remnant,” says Cerny. “The fact that the system appears intact leads to two equally interesting possibilities. Either UMa3/U1 is a tiny galaxy stabilized by large amounts of dark matter, or it’s a star cluster we’ve observed at a very special time before its imminent demise.”

If astrophysicists can confirm that the galaxy has dark matter, that would be a big deal. It would be more evidence in support of the Lambda Cold Dark Matter (CDM) model, the leading theory for dark matter and the Big Bang. CDM predicts that as the Milky Way formed, its gravity attracted large numbers of dwarf galaxies, much more than found so far. If this is one of them, and if the others are as difficult to detect as UMa3/U1, it supports the CDM.

But for the researchers behind the discovery, there’s more to it than just dark matter. They’ve found something unusual that’s difficult to detect. Are there more of them out there?

The ESA’s Gaia mission has found many dwarf galaxies and globular clusters in the Milky Way’s halo. This image from the mission’s second data release shows 75 globular clusters (blue) and 12 nearby dwarf galaxies (red). However, deeper observations are needed to understand the nature of the dwarf galaxies. Image Credit: ESA/Gaia/DPAC; Map and orbits: CC BY-SA 3.0 IGO LICENCE CC BY-SA 3.0 IGO or ESA Standard Licence

“Whether future observations confirm or reject that this system contains a large amount of dark matter, we’re very excited by the possibility that this object could be the tip of the iceberg – that it could be the first example of a new class of extremely faint stellar systems that have eluded detection until now,” says Cerny.

As for its origins, there are really only two options. It either formed in situ or was accreted by the Milky Way. Astronomers use metallicity and orbit to determine a dwarf galaxy’s origins, but in this case, neither measurement showed clearly that it formed in situ.

Only further observations will constrain its origins, but as it stands, the authors are leaning toward accretion. “We favour a scenario where UMa3/U1 was accreted onto the Milky Way halo,” they write in their conclusion. That scenario also supports the Lambda CDM model.

Its fate is similarly unclear. So far, it hasn’t been torn apart, which signals the presence of dark matter. But if it doesn’t have dark matter, it may be on the verge of being destroyed. We’ll have to wait and see.

For now, the object has an uncertain past and an uncertain future. But whatever it ends up being classified as, it’s something new, and that means it’s a challenge.

“This discovery may challenge our understanding of galaxy formation and perhaps even the definition of a ‘galaxy,'” says Smith.

Evan Gough

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