If our knowledge of galaxy structures was limited to the Milky Way, we'd get a lot of things wrong. The Milky Way, it turns out, is unusual. It's got a smaller central black hole than other galaxies its size; its halo is also smaller and contains less of the heavier elements. Fortunately, we've now looked at enough other galaxies to know that ours is a bit of an oddball. What's been less clear is why.
Luckily, a recent study provides a likely answer: compared to most galaxies, the Milky Way's had a very quiet 10 billion years or so. But the new study suggests we're only a few billion years from that quiet period coming to an end. A collision with a nearby dwarf galaxy should turn the Milky Way into something more typical looking—just in time to have Andromeda smack into it.
Incoming
The researchers behind the new work, from the UK's Durham University, weren't looking to solve the mysteries of why the Milky Way looks so unusual. Instead, they were intrigued by recent estimates that suggest one of its satellite galaxies might be significantly more massive than thought. A variety of analyses have suggested that the Large Magellanic Cloud has more dark matter than the number of stars it contains would suggest. (Its stellar mass is estimated to only be five percent of the Milky Way's.)
Using some updated values, the researchers simulated the Local Group of galaxies, including the Milky Way, Andromeda, and the Large Magellanic Cloud. While the Large Magellanic Cloud is currently moving away from our galaxy, the simulation suggested its on a gradually arcing orbit that will bring it back towards us. And, a bit under three billion years from now, it will end up merging with the Milky Way.
That's actually about a billion years earlier than the Andromeda galaxy (which is larger than the Milky Way) was set to run into us head-on. But the interactions between the Milky Way and the Large Magellanic Cloud will mean that the Milky Way won't be quite where we'd expected it to be at the time. As a result, the Andromeda-Milky Way smash up will happen a billion years later than expected, and it will be somewhat oblique.
Outcomes
So what does the product of a Milky Way-Large Magellanic Cloud smash up look like? Normally, the way to handle that question would be to perform a model of the collision starting with the actual conditions we observe now. But the researchers went a somewhat different route, taking advantage of a system that incidentally ran multiple model runs of similar collisions. The EAGLE project starts out with the conditions at the formation of the Cosmic Microwave Background, and it simulates the evolution of the Universe over billions of years that follow. EAGLE covers a volume that's big enough to include about 10,000 galaxies, and at least some of them underwent collisions that look a lot like the one we expect the Milky Way to experience.
One thing that the EAGLE simulation makes clear is that the unusual properties of the Milky Way—small black hole and halo, a metal-poor halo—are the product of a relatively collision-free history. At most, these galaxies could have experienced a couple of collisions with very small dwarf galaxies over the last 10 billion years or so. Conveniently, that's consistent with data from the Gaia mission, as tracks of stars indicate the Milky Way's last major collision was at least 8 billion years ago.
A collision with the Large Magellanic Cloud would, in essence, fix all of this. It would create disturbances in the core of the Milky Way that would strip some momentum out of the gas there, allowing it to fall inwards towards the center, feeding the black hole there. Most similar collisions in the EAGLE simulation saw the central black holes increase by at least a factor of 2.5. (Many of them also resulted in a strong brightening and jet formation at the black hole itself, creating an active galactic nucleus.)
About 20 percent of the stars in the Large Magellanic Cloud would end up being ejected into intergalactic space, and a few from the Milky Way share that fate. But many more stars from the center of the Milky Way end up being ejected into the halo. The center of the Milky Way is rich in heavier elements, and this process makes the Milky Way look more typical in that regard. The combination of stars from the galaxy center and those stripped from the Large Magellanic Cloud also increases the visible mass of the halo, handling that feature as well.
Since none of the starting conditions precisely match those of the Local Group of galaxies, the EAGLE simulation provides a range of possibilities for the future of the Milky Way. As such, we probably still will want to build a model that specifically runs forward starting from the current conditions. Then, all we'd have to do is wait about three billion years to see how well the model did.
Monthly Notices of the Royal Astronomical Society, 2019. DOI: 10.1093/mnras/sty3084 (About DOIs).
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