The first evidence of dark matter is that galaxies behave as if they contain more matter than the mass of their stars can explain. Since then, we have found many other indications that the universe is swimming with matter that we can only deduce from the effects of gravity. Original evidence that galaxies of dark matter structures survive.
Until that awkward moment in 2018 when we seemed to find whereabouts Almost a galaxy without dark matter. Although there were doubts about this initial discovery, the researchers eventually found a second galaxy that was very similar. This is a problem, because we don’t know how galaxies like this form.
Now, researchers have announced that while running models of galaxy formation, a number of dark matter-poor galaxies naturally emerge. And by tracing the model back to see how the galaxies came to be, researchers can provide an explanation for these mysteries.
Join the dark side
Galaxies and dark matter are difficult to separate. Early universe models showed that dark matter coalesced into filaments (which… We found) which converges at the node having the higher material density. These nodes, in turn, attract visible matter, causing galaxy clusters to form around them. These galaxies would then combine much of the dark matter, which has shaped its growth through the process of merging and structure to this day. Therefore, it is difficult to understand how a galaxy could end up with so little dark matter.
As a result, when the discovery of dark matter-poor galaxies was first announced, doubts were high. Much of this uncertainty focuses on the distance from the galaxy to us, which is critical for estimating its mass and thus its dark matter content. But the same team identified a second galaxy that appears to have many of the same characteristics as the first problem.
Another reason for suspicion is because we do not know how this type of galaxy formed. We have done a lot of modeling of galaxy formation, including models that begin before filaments of dark matter are formed. If dark matter -poor galaxies were a natural part of the evolution of the universe, we would probably see models produce them. But there was no news of anyone present.
So, we have two eccentric-looking galaxies. We have questions about their actual characteristics and no clue as to what they look like. This is not a pathological condition.
updated model
But the solution may come from a different researcher trying to do something completely different. What they did was model the evolution of galaxies using an advanced software package called FIRE-2 (for “Reactions in Realistic Environments”). Although they didn’t say what they were trying to see, they did say they weren’t dark matter-poor galaxies: “We note that we didn’t expect this to happen beforehand (that is, our simulations weren’t originally designed for this purpose).”
Regardless of the goal, running the model yielded seven galaxies that were virtually devoid of dark matter. Researchers named these names after the seven Cherokee clans, meaning things like Blue, Deer, and Wolf but also things like Long Hair and Wild Potato. The last two sentences are set for some unexpected sentences in astronomy papers, such as “For example, the bird displays an S-shaped tail of low brightness, while Long Hair appears undisturbed.”
However, what’s useful here is that the model already stores information about the intermediate states, which means we can run simulations in time and see how these imaginary galaxies end up without dark matter. But what has become clear is that the galaxies didn’t start that way. The seven are, at some point in the simulation, like typical small galaxies with a natural supplement of dark matter.
Instead, each small galaxy loses its dark matter through what you can see as a failed merger with a much larger galaxy. From a simulation perspective, which relies on metrics such as mass distribution within the galaxy, the researchers define this as a “close encounter.” But from the perspective of what a failed merger would look like if we imaged it as it happened, most of the stars of the smaller galaxy would be in the region occupied by the star of the larger galaxy. In short, the tiny galaxy looked like it was being swallowed up before somehow appearing on the other side.
But the tiny galaxy doesn’t seem to have changed. While up to half of its stars appear, nearly all of the gas and dark matter ends up in larger galaxies.
If the statistics shown in this model can be applied generally, there may be more galaxies lacking dark matter than the two galaxies we have discovered so far. The research team estimates that up to 30% of the massive galaxies in the universe must orbit nearby. Sounds like an invitation to observing astronomers to get busy and discover more.
Natural Astronomy, 2019. DOI: 10.1038 / s41550-021-01598-4 (About DOI).
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