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The Milky Way on the horizon. Credit: NASA

Dark matter could be powering a galaxy that orbits the Milky Way until they collide

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May 1, 2021, 9:53 PM EDT

Have you ever looked up into the darkness and wondered what is on the edge of the Milky Way? Stars, an orbiting galaxy, and an almost unfathomable amount of dark matter.

Astronomers who were virtually navigating through the furthest reaches of our galaxy through data from the NASA NEOWISE and ESA Gaia missions have come up with a new map of what is floating out there. In what is known as the galactic halo, something appears to be pulling at the Large Magellanic Cloud (LMC), a much smaller galaxy that is still only on its first orbit around the Milky Way. That something is also leaving a wake of stars trailing behind it as the LMC sails on. The unseen forces that are probably doing this come from the Dark Side.

Darth Vader may have not had much use for dark matter, but Nicolas Garvito-Camargo of the University of Arizona, who recently led the study, coauthor Rohan Naidu of Harvard, and their colleagues believe it could be the reason that the LMC will eventually crash into the Milky Way—something the Death Star never got to do.

“Stars in the outer regions of the galaxy were observed moving faster than expected given the amount of mass observed in the stars,’ Garvito-Camargo told SYFY WIRE. “This was one of the first pieces of evidence that there should be more mass in the galaxy than what is observed.”

Because there is more mass in our galaxy than they could see, the astronomers came to the conclusion that the rest of the mass must be coming from the invisible stuff that is dark matter. What makes dark matter truly invisible (instead of just hidden) is that it emits no light and cannot be illuminated. This kind of matter was first hypothesized about when the Vera Rubin Observatory found a disconnect between the mass of the stars in the galactic halo and the speed at which they were moving. When it measured the rotation curve of neighboring galaxy Andromeda, that was one of the first instances in which scientists suspected dark matter.

 

Andromeda’s strange rotation curve meant the masses and speeds of orbiting stars in the galaxy, compared to their radial distance from its center, did not add up. The rotation curve of the Milky Way turned out to have the same problem. Measuring its rotation curve and quantifying the invisible matter gave an idea of how much dark matter they were dealing with in the galactic halo. Stars also helped light the way to what could not be seen. Garvito-Camargo and his team used NEOWISE data to measure starlight in a specific infrared wavelength that gave away the most massive stars in the halo.

“With this information, we could distinguish which stars were the giants, and once we selected them, we could calculate what would be the distance to those stars to see the apparent brightness we were measuring, not only in the infrared but also in those from Gaia,” he said. “We could do this since we have models that tell us what the brightness of those stars is at a given distance.”

By mapping the galactic halo, the scientists were also given the chance to test out the properties of its dark matter by using the Cold Dark Matter Theory. Assuming the dark matter is cold means that it moves slowly as opposed to being actually frozen. Because galaxies and galaxy clusters have vast quantities of this slow, dense dark matter, its gravity holds them together and keeps stars, planets and other objects from fling everywhere in space. It would probably be impossible to send spacecraft anywhere if it weren’t for the gravitational grip of dark matter keeping things where they should be.

“Using another dark matter theory would have an impact on the morphology of the wake,” said Garvito-Camargo. “We can make the analogy to viscosity. More viscous fluid would behave differently even if it is exposed to the same perturbations. Similarly, some dark matter models would react differently to the LMC. As a result, the morphology of the wake would be different.”

Because the dark matter in the galactic halo generates such an immense amount of gravity, it will keep pulling the Large Magellanic cloud closer and closer to the Milky Way with each subsequent orbit until they merge in another 2 million years or so.

At least we can safely say none of us will be around by then.