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With relief, researchers find an explanation for dark-matter-poor galaxies

The slightly hazy, light patch in the middle of the image is the DF2 galaxy, so diffuse that other galaxies are clearly visible behind it.
Enlarge / The slightly hazy, light patch in the middle of the image is the DF2 galaxy, so diffuse that other galaxies are clearly visible behind it.

The first evidence for dark matter were galaxies that behaved as if they had far more matter than could be accounted for by the mass of their stars. Since then, we’ve found many other indications that the Universe is swimming with matter that we can only infer from its gravitational effect. And the original evidence that dark matter structures galaxies continued to hold.

Until that awkward moment in 2018 when we appeared to discover the existence of a galaxy with almost no dark matter. While there was skepticism regarding this initial finding, researchers eventually spotted a second galaxy that was very similar. Which was a problem, given that we had no idea how galaxies like this could form.

Now, researchers have announced that, while they were running a model of galaxy formation, a number of dark-matter-poor galaxies naturally popped out. And, by tracing the model back to see how those galaxies arose, the researchers are able to offer an explanation for these enigmas.

Join the dark side

Dark matter and galaxies are difficult to separate. Models of the early Universe suggest that dark matter coalesced into filaments (which we’ve detected) that met at nodes that held higher densities of the material. These nodes, in turn, attracted visible matter, causing clusters of galaxies to form around them. Those galaxies would then incorporate a lot of the dark matter, which shaped their growth through mergers and structure to the present day. So, it’s difficult to understand how a galaxy could end up with very little dark matter.

As a result, when the discovery of an apparently dark-matter-poor galaxy was first announced, skepticism was significant. Much of that skepticism was focused on the distance from us to the galaxy, which was critical to estimating its mass and thus its dark-matter content. But the same team identified a second galaxy that seems to share many of the same properties as the first troublemaker.

The other reason for skepticism was that we didn’t have any idea how this sort of galaxy might end up forming. We’ve done plenty of modeling of galaxy formation, including models that started before the formation of dark-matter filaments. If dark-matter-poor galaxies were a normal part of the Universe’s evolution, then we probably should have seen the models produce one. But there was no report of one showing up.

So, we were left with a couple of apparent oddball galaxies. We have questions about their actual properties and no hint of how they could have ended up looking like they do. This is not a satisfying situation.

Updated model

But a solution may have come from different researchers trying to do something else entirely. What they were doing was modeling galaxy evolution using a state-of-the-art software package called FIRE-2 (for “Feedback In Realistic Environments”). While they don’t say what they were trying to look at, they do say it wasn’t dark-matter-poor galaxies: “We note that we did not expect this to occur a priori (that is, our simulation was not originally designed for this purpose).”

Regardless of intent, however, a run of the model produced seven galaxies that were nearly devoid of dark matter. The researchers named these after seven Cherokee clans, which meant things like Blue, Deer, and Wolf but also things like Long Hair and Wild Potato. The latter two made for some unexpected sentences in an astronomy paper, such as “For instance, Bird exhibits ‘S-shaped’ low-surface-brightness tails, whereas Long Hair seems to be undisturbed.”

In any case, the handy thing here is that the model saved information about intermediate states, meaning we could run the simulation back in time and see how these fictional galaxies ended up without dark matter. What became clear, though, was that those galaxies didn’t start out that way. All seven had, at one point in the simulation, looked like typical small galaxies with normal complements of dark matter.

Instead, each small galaxy lost its dark matter through what you can view as a failed merger with a much larger galaxy. From the perspective of the simulation, which relies on measures like the distribution of mass within a galaxy, the researchers define these as “close encounters.” But from the perspective of what a failed merger would look like if we imaged it while it was happening, most of the small galaxy’s stars would be inside the area occupied by the stars of the larger galaxy. In short, the small galaxy looked like it was being swallowed up before somehow emerging on the other side.

But the small galaxy did not emerge unchanged. While up to half of its stars emerged, almost all of the gas and dark matter ended up deposited in the larger galaxy.

If the statistics seen in this model are generally applicable, there may be many more of these dark-matter-deficient galaxies out there beyond the two we’ve found so far. The research team estimates that up to 30 percent of the massive galaxies in the Universe should have one orbiting nearby. Which sounds like an invitation for the observational astronomers to get busy and find some more.

Nature Astronomy, 2019. DOI: 10.1038/s41550-021-01598-4  (About DOIs).

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