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Scientists have found a galaxy with almost no dark matter and we have finally solved the Leading Arm mystery!

Host: Caitlin Hofmeister

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[ ♪ Intro ].

Galaxies are all over the news this week! Galaxies near, galaxies far, galaxies wherever you are … and, yes, that does mean the Milky Way.

But first, in a paper published this week in the journal Nature, an international team of astronomers discovered that a distant galaxy has almost no dark matter, throwing our expectations straight out the window. Dark matter isn’t so much dark as it is invisible to all current methods of detection. It doesn’t interact with light at all, so we can’t see it, but it does have mass, so we can see the effects of its gravitational pull.

In most galaxies, dark matter is more abundant than regular, or baryonic, matter. In our own Milky Way it outnumbers baryonic matter roughly 30 to 1, but in dwarf galaxies, that ratio is over 10 times greater! So to find a small galaxy with almost no dark matter at all would be super weird.

So, obviously, we found one. It’s called NGC1052–DF2, although we’ll just call it DF2 for short. To figure out how much dark matter it has, astronomers needed to compare the mass of the matter we can see to the galaxy’s total mass.

They were able to determine DF2’s stellar mass, or how much of its mass comes from stars, based on its brightness and distance. The team estimated that it’s around 65 million light-years away, which produced a stellar mass of about 200 million times the mass of the Sun, which is roughly 250 times less than that of the Milky Way. Then they calculated the mass of the halo surrounding the galaxy, which should, theoretically, be chock full of dark matter.

To do that, they measured the velocities of 10 different star clusters at distances between 1300 and 25,000 light-years from the center. Since a more massive galaxy will make stars orbit faster, they used those clusters’ velocities to calculate the total mass for the whole galaxy. Then they subtracted the stellar mass they’d already accounted for, which left them with the mass of the dark matter halo.

Which turned out to be less than 150 million times the mass of the Sun. Now, that’s not zero dark matter, but current models say the number should be closer to 60 billion solar masses — 400 times more than what we’re seeing! Since it’s such a unique oddity, we still don’t know how a dark matter deficient galaxy like DF2 came to be.

The team proposed a few ways it might have formed from regular matter that wouldn’t have taken much dark matter with it. For example, from gas that was flung out of merging galaxies, or that was streaming toward a neighboring galaxy but ended up splitting off. Until we find more galaxies like this one, it’ll be tough to come up with a solid origin story.

But the discovery does help show that dark matter and baryonic matter are not inexorably linked to one another — in other words, the amount of one does not determine the amount of the other. And, it helps put another nail in the coffin for hypotheses that dark matter isn’t real, and that what we call dark matter is actually just our own physics equations being wrong. Because if that were the case, we wouldn’t expect to find random galaxies with very little of it.

As for when we’ll finally pin down what exactly dark matter is, only time will tell. Much closer to home, astronomers have finally solved a mystery involving a galactic tug of war and cannibalism on the outskirts of the Milky Way. The Large and Small Magellanic Clouds are irregular dwarf galaxies about 160,000 and 200,000 light-years from our own.

They both orbit the Milky Way, but as they do so, they also orbit one another. The gravity involved in that system means that one of the Clouds has ripped away some of the other’s gas, and sent it spiraling toward us in a fragmented arc roughly half the length of the Milky Way! The question is: which Cloud is siphoning gas away from the other?

That’s been a mystery for some time now — although admittedly not as long as the arc has been around, since it’s roughly 2 billion years old. Obviously we weren’t around back then to ask these kinds of questions. Thanks to a paper published last month in the Astrophysical Journal, we may finally have an answer.

Astronomers call this arc of gas the Leading Arm, because it’s “leading the motion” of the Magellanic Clouds. It kinda looks like this arm starts out in the Large Magellanic Cloud. But to confirm its real galactic parent, the team used the Hubble Space Telescope to analyze some of its composition, and compare it to both dwarf galaxies.

Specifically, they looked at the light from seven quasars, incredibly bright cores of galaxies powered by supermassive black holes. Based on where they’re each located in space, the light from these quasars had to travel through one of the Magellanic Clouds or the Leading Arm on its way to us. And depending on their composition, the molecules in the Clouds and Arm would have absorbed different wavelengths of the light as it passed through.

By analyzing which types of light were absorbed, the team was able to match the Arm’s composition with … the Small Magellanic Cloud. Large Magellanic Cloud, you are not the father! Or mother.

But you did kind of gravitationally tear off part of your galactic partner and allow the. Milky Way to cannibalize it and make new stars, so it’s complicated. Of course solving this mystery wasn’t just to satisfy curiosity.

Understanding how gas falls - or accretes - into galaxies is an important step toward better models of how galaxies grow and evolve. But most galaxies are too far away for us to detect the light we’d need to study. So the fact that we have an example of this galactic gas accretion right on our own front porch is a huge benefit.

Between that and DF2’s missing dark matter, we’ve got all kinds of galactic mysteries to solve! Thanks for watching this episode of SciShow Space News! For more on galaxies with weird amounts of dark matter, you can check out our episode on Galaxy X, which is almost entirely made up of dark matter. [ ♪ Outro ].