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If our knowledge of galactic structures was limited to the Milky Way, many things would be wrong. It turns out that the Milky Way is unusual. It has a smaller central black hole than other galaxies; its halo is also smaller and contains fewer heavier elements. Fortunately, we have now examined enough galaxies to know that ours is a bit strange. What was less clear, that is why.
Fortunately, a recent study provides a probable answer: compared to most galaxies, the Milky Way has had a very quiet period of 10 billion years. But the new study suggests that we will be only a few billion years from the end of this quiet period. A collision with a nearby dwarf galaxy should turn the Milky Way into a more typical appearance – just in time for Andromeda to hit her.
incoming
The researchers behind the new work, from the University of Durham in the UK, were not trying to solve the mysteries of why the Milky Way looked so unusual. Instead, they have been intrigued by recent estimates suggesting that one of its satellite galaxies could be much more massive than expected. Various analyzes have suggested that the Great Magellanic Cloud contains more dark matter than the number of stars it suggests. (It is estimated that its star mass is only five percent of that of the Milky Way.)
Using updated values, the researchers simulated the local group of galaxies, including the Milky Way, Andromeda, and the Large Magellanic Cloud. While the Great Magellanic Cloud is moving away from our galaxy, the simulation has suggested it in an arc orbit that will bring it back to us. And, in a little less than three billion years, it will eventually merge with the Milky Way.
It's actually about a billion years earlier than the Andromeda galaxy (which is larger than the Milky Way) was going to face us head-on. But the interactions between the Milky Way and the Great Magellanic Cloud will mean that the Milky Way will not be exactly as we hoped to be at that time. As a result, the destruction of the Andromeda-Milky Milky Way will occur a billion years later than expected and will be somewhat oblique.
The results
So, what does the product of a Magellanic cloud of Smooth Milky Way-Large look like? Normally, the best way to deal with this question would be to make a collision model based on the actual conditions observed. But the researchers took a somewhat different route, taking advantage of a system that, incidentally, ran several similar crash model runs. The EAGLE project begins with the conditions of formation of the cosmic microwave background and simulates the evolution of the Universe over billions of years. EAGLE covers a volume large enough to include about 10,000 galaxies, and at least some of them have suffered collisions that are very similar to what we expect from the Milky Way.
The EAGLE simulation clearly shows that the unusual properties of the Milky Way – small black hole and halo, a metal halo – are the product of a relatively collision-free story. At most, these galaxies may have experienced some collisions with very small dwarf galaxies in the last 10 billion years or so. This is consistent with data from the Gaia mission, as traces of stars indicate that the last major collision occurred in the Milky Way occurred at least 8 billion years ago.
A collision with the great magellanic cloud would solve all this. This would create disruptions in the heart of the Milky Way, which would reduce the momentum of the gas and allow it to fall toward the center, thereby fueling the black hole. In most similar collisions in the EAGLE simulation, the central black holes were multiplied by at least 2.5. (Many of them also resulted in strong lightening and jet formation at the black hole itself, creating an active galactic core.)
About 20% of the stars in the Great Magellanic Cloud would eventually be ejected into an intergalactic space, and some of the Milky Way would share that fate. But many other stars in the center of the Milky Way end up being ejected into the halo. The center of the Milky Way is rich in heavier elements, which gives it a more typical appearance. The combination of stars in the center of the galaxy and those extracted from the great Magellan cloud also increases the visible mass of the halo, while retaining this characteristic.
Since none of the initial conditions match exactly that of the local galaxy group, the EAGLE simulation offers a range of possibilities for the future of the Milky Way. As such, we will probably still want to build a model that works specifically from the current conditions. Then we would have to wait about three billion years to see if the model worked well.
Monthly Notices from the Royal Astronomical Society, 2019. DOI: 10.1093 / mnras / sty3084 (About the DOIs).
