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Andromeda collided with galactic siblings billions of years ago



A new study indicates that the stellar halo and stellar evolution of our neighboring galaxy are due to a large collision that ended 2 billion years ago

A large M32 galaxy crashed into the Andromeda galaxy billions of years ago Evolution of both galaxies forever. The new study – published in Nature Astronomy – suggests that fusion affects the halo of stars and globular clusters around the Andromeda disk, as well as the pace of star formation in the galaxy.

The Andromeda galaxy with its strangely compact satellite galaxy, M32 (Fuzzball under Andromeda's disk). Astronomers now believe that M32 could be the remnants of a massive galaxy that interacted with andromeda between 5 billion and 2 billion years ago.
Lorenzo Comolli

Andromeda is the largest member of the local group of galaxies near the Milky Way. a spiral galaxy that is about 2.5 million light-years away. Its proximity motivates researchers Richard D & # 39; Souza (University of Michigan) and Eric Bell (Vatican Observatory) to understand their evolution. The researchers have obtained information from two independent large-scale simulations, Illustris and C13 on how Andromeda galaxies intermingle. Specifically, they were interested in how Andromeda's star disk got so thick: it's 2600 light-years high, eight times thicker than the thin disk of the Milky Way, though it's only twice as wide.

The simulations showed that such a massive star disk only emerges after a large galactic collision that stretches for billions of years. Andromeda's star disk has a high metallicity or a wealth of elements heavier than hydrogen and helium, suggesting that it was a massive, metal-rich galaxy bouncing against Andromeda. The two had started interacting 5 billion years ago, with disruptive effects lasting up to 2 billion years ago.

The researchers conclude that M32, a strangely compact galaxy in orbit around Andromeda, is probably what's left of Andromeda. The researchers call the mother galaxy M32p and suggest that it is similar in mass to the Milky Way, with a low-mass bulge in its center. Researchers have been wondering for years why M32 contains so many stars in such a small volume, says D & # 39; Souza. If a collision reformed this galaxy long ago, it could explain many of its unique features.

The most amazing result, D & Souza adds, is that Andromeda's plate survived the massive merger; Many previous studies suggested that such crashes would lengthen a spiral galaxy into an elliptical galaxy. However, observations show that the disk and the bulge of the galaxy were present before the fusion, so that our neighboring galaxy maintained its spiral structure during the collision. Puragra Guha Thakurta (University of California, Santa Cruz), who was not involved in the study, explains that not every major fusion produces an elliptical galaxy. The end result depends on the exact mass ratio of the two merging galaxies and the geometry of the collision.

Additional evidence for the historical collision comes from a gigantic stream of metal-rich stars discovered in Andromeda's halo in the early 2000s. Perhaps this property is a debris field that is created in the aftermath of the crash, the researchers suspect. Other features indicate a greater disturbance. For one thing, about one-fifth of Andromeda's stars formed about 2 billion years ago – the after-effects of fusion could have triggered their birth.

D & Souza adds that such a major merger might have disrupted the history of the entire local group, but more research in this direction is needed. "It's like the missing family member nobody wants to talk about," he says. "It's a big surprise that there was something so big in our area."

Puragra Guha Thakurta (University of California, Santa Cruz), who was not involved in the study, says the methodology of the study is solid. However, there is strong evidence against the idea that M32 is the remnant of a major merger – behind M32 there is a huge stream of stars and gas, but nothing is the same. "It is generally believed that the tidal debris of a fusion event is approximately symmetric to the merger residue," he explains.

He also notes that several aspects of the scenario include "significant uncertainty". These include the mass of galaxies, the motion of their stars, their history of star formation and other factors that would affect the course of the merger. That is, "It would be nice to be able to explain a number of phenomena … with a single major fusion event," he says.

While it is unclear whether M32 represents the remnants of a large merger, D & S Souza and Bell claim that their work makes a strong case that has some massive interaction with Andromeda. Like a company trying to grow up quickly, D & # 39; Souza says that a galaxy looking for a big mass jump is likely to need a fusion with another galaxy of similar mass. He adds that more observations of Andromeda's stellar motions and its composition will help shore up the case.


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