The Andromeda galaxy is destined to collide and merge with the Milky Way a few billion years into the future and now researchers have uncovered some clues suggesting that it has done this sort of thing before. The two galaxies might have had a large sibling in the past that was destroyed and eaten by Andromeda.

As reported in Nature Astronomy, researchers constructed a model of Andromeda to establish how merger events have shaped the galaxy. Big galaxies like ours and Andromeda are expected to have cannibalized hundreds of smaller galaxies, and the leftovers are seen in ribbons that wrap around the major galaxy. These streams of stars are the only evidence we have of some of these events.

Astronomers looked at the ribbons around Andromeda and found a more complex picture. There is a halo of stars, almost invisible, which is larger than Andromeda. There is also a stream of stars, as well as a peculiar compact galaxy called M32. The simulation suggests that M32 is what remains of a much bigger galaxy destroyed by Andromeda.

“It was a ‘eureka’ moment. We realized we could use this information of Andromeda’s outer stellar halo to infer the properties of the largest of these shredded galaxies,” lead author Dr Richard D’Souza, from the University of Michigan, said in a statement.

M32p, as this potential predecessor has been nicknamed, used to have a mass of 25 billion Suns before the collision with Andromeda took place 2 billion years ago. The merger did not massively change the shape of Andromeda but brought the galaxy a new age of star formation, leading to many new stars being born.

“The Andromeda galaxy, with a spectacular burst of star formation, would have looked so different 2 billion years ago,” co-author Professor Eric Bell added. “When I was at graduate school, I was told that understanding how the Andromeda galaxy and its satellite galaxy M32 formed would go a long way towards unraveling the mysteries of galaxy formation. Astronomers have been studying the Local Group – the Milky Way, Andromeda and their companions – for so long. It was shocking to realize that the Milky Way had a large sibling, and we never knew about it.”

The team believes that this method can be used on other galaxies. They hope that this approach can help astronomers better understand the important role that merger events play in the evolution of galaxies.