The Andromeda galaxy has long been considered the Milky Way’s “sister,” due to the the pair's comparable age, shape, and shared future as a mega-galaxy. And though Andromeda boasts about one trillion stars—over twice the Milky Way’s stellar population—there has been a longstanding assumption that the galaxies are around the same weight.
But a team of astronomers based out of the University of British Columbia have upended that consensus in a new paper. The team concluded that Andromeda is almost two times as heavy as our own galaxy, mostly because it has stockpiled double the amount of dark matter of the Milky Way. The full paper is available in the July edition of the Monthly Notices of the Royal Astronomical Society.
It’s unclear how Andromeda got its spiraled hands on so much dark matter, but co-author Yin-Zhe Ma has a few speculative guesses. “In the early universe, the universe was almost smooth, but there were random fluctuations in one place or another,” he told to me in a phone interview.
“These regions might attract more materials, giving Andromeda a slightly stronger gravity center. It could be a random process,” he said. “It could also be [that Andromeda] formed slightly earlier than the Milky Way, so it might have had more time to accumulate material.”
Ma and his colleagues were able to model the relative weight of the galactic sisters by examining the smaller satellite galaxies surrounding them. “We built computer models to simulate the two galaxies as a dumbbell structure in an expanding universe,” he said.
“The model showed the movement of small satellite galaxies around the larger galaxies. We measured the speed, position, and motion of these satellite galaxies to infer the structure and mass of the Milky Way and Andromeda.” And voilà—these simulations revealed that Andromeda is packing dark matter like a champ.
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