A team of astronomers has solved an intergalactic crime: the murderer was … the Andromeda galaxy. Experts have found the body of the victim. It is a massive sister galaxy to the Milky Way that was crushed and cannibalized by this spiral galaxy with a diameter of 220,000 light years that we know as Andromeda.
The discovery, published in the journal Nature Astronomy, seems to be part of a detective plot, but that is how they have solved the ‘astronomical case’. Researchers carefully studied a mysterious star trail, a shrunken galactic body, and the faint halo of stellar debris around Andromeda to piece together the facts: about 2 billion years ago, around the time they first formed in the world. Earth eukaryotes, our most basic ancestors, there was a third large galaxy in the neighborhood. But Andromeda crushed it, swallowed it and threw its corpse into the darkness of the cosmos.
Scientists had long suspected that Andromeda might have undergone a merger in the recent past, says Amanda Moffett, an astronomer and galaxy evolution expert at Vanderbilt University, who was not involved in the study. What is new and surprising about this find is the idea that M32, a small dwarf galaxy orbiting Andromeda, was actually the corpse of a much larger galaxy destroyed in that merger.
The Milky Way Sister Trail
To answer that question, the researchers ran some computer simulations of the galactic interaction. They found that, with the correct parameters, a merger of galaxies 2 billion years ago could produce the clues we have today in Andromeda: a long stream of stars stretching out into the universe, an extra thick fog of loose stars around Andromeda, a bunch of stars in Andromeda that formed 2 billion years ago, and the core of that lost galaxy still floating nearby, in the shape of M32.
Typically, dwarf galaxies like M32 are made up of stars that formed from the same small group of material. Larger galaxies like the Milky Way and Andromeda tend to have stars with a much wider age range. But the stars in M32 are like the stars in a larger galaxy, scattered in age over billions of years.
The finding may explain M32’s odd age range, which is that it was once a much larger galaxy that ended up shrinking. Thus, this work solves two mysteries at the same time: what happened to the Andromeda victim, and why M32 is such an atypical dwarf galaxy.
“The Andromeda galaxy, with a spectacular burst of star formation, would have looked so different 2 billion years ago … When I was in graduate school, I was told to understand how the Andromeda galaxy and its satellite galaxy would form M32 would be a great advance to unravel the mysteries of galaxy formation “, says Eric Bell, co-author of the work.
This discovery not only helps scientists learn more about the universe around us, how we got here, and how galaxies evolve, but it also tells us how ‘dangerous’ Andromeda is. While our neighbor won’t actually be coming for us in the near future, it will happen eventually, and we better be prepared for it.
Reference: Richard D’Souza et al, The Andromeda galaxy’s most important merger about 2 billion years ago as M32’s likely progenitor, Nature Astronomy (2018). DOI: 10.1038 / s41550-018-0533-x
