The Milky Way once had a massive galactic sibling that was shredded and consumed by our closest neighbour, Andromeda, scientists have discovered.
The dramatic sequence of events, dating back 2bn years, was reconstructed through a detailed survey of stars in the faint halo surrounding the Andromeda galaxy.
Many had assumed the contents of the halo had been acquired piecemeal, as Andromeda swallowed up hundreds of small galaxies over billions of years. However, the latest research uncovered a trail of evidence showing the bulk of the stars could be traced to a minor satellite galaxy of Andromeda known as M32.
This once ranked alongside the Milky Way and Andromeda in size, the paper concludes, but after its outer layers were cannibalised it was reduced to a small, dense core, like the pit of a plum.
“It was shocking to realise that the Milky Way had a large sibling and we never knew about it,” said co-author Eric Bell, a professor of astronomy at the University of Michigan. “Astronomers have been studying the Local Group – the Milky Way, Andromeda and their companions – for so long.”
At about twice the size of the Milky Way, Andromeda is the biggest galaxy in our local group. M32 would have been the third biggest in its day.
Scientists have long known that the outer halo regions of galaxies contain debris left over from smaller cannibalised galaxies.
“It’s kind of like a child eating dinner, and then looking on the floor afterwards and finding breadcrumbs all around,” said Richard D’Souza, a postdoctoral researcher at the University of Michigan. “You know what’s been eaten.”
Andromeda’s halo has been of particular interest as it is close enough to study in detail, and the huge streams of stars and asymmetric features that are visible suggest the galaxy has been witness to some dramatic mergers.
A forensic survey of the halo suggested there were far more stars than could be explained through the acquisition of very small galaxies. And the distribution of the ages of stars – almost all being at least 2bn years old – pointed to a single large galaxy having been swallowed around this time point.
The theory is backed up by a massive stream of stars trailing out behind Andromeda, which the scientists believe can be tracked back to this event. And it could help explain M32’s unusual composition, which has remained a source of puzzlement until now.
“M32 is a weirdo,” Bell said. “While it looks like a compact example of an old, elliptical galaxy, it actually has lots of young stars. It’s one of the most compact galaxies in the universe.”
Michael Merrifield, professor of astronomy at the University of Nottingham, who was not involved in the research, said: “The possible explanation of M32 as the undigested core of this lost galaxy is certainly a nice idea, as its existence is a bit of a mystery.”
The findings could provide some insight into our own galaxy’s eventual fate. The Milky Way and Andromeda are approaching each other at about 400,000km per hour and will smash together in about 4bn years time to form a giant elliptical galaxy. “We will be shredded and be part of the galactic halo,” said D’Souza.
The findings are published in Nature Astronomy.