Origin of ‘weirdo’ galaxy helps reveal Milky Way’s long-lost sibling

23 Jul 2018

Image: Alex Mit/Shutterstock

The shredding and cannibalisation of a massive galaxy aeons ago by Andromeda led to the creation of a long-lost sibling of the Milky Way.

For years, astronomers have been trying to solve an almighty cosmic mystery located near Andromeda, the nearest galaxy to our own Milky Way.

Observations of the region of space showed the existence of a satellite galaxy around Andromeda called M32, but no one had any idea how it came about, until now.

In a paper published to Nature Astronomy, a team of scientists from the University of Michigan revealed that 2bn years ago, Andromeda shredded and cannibalised a massive galaxy dubbed M32p, leaving behind an almost invisible halo of stars larger than Andromeda itself.

To get a sense of how enormous M32p was, the team estimates that it was at least 20 times larger than any galaxy that merged with the Milky Way over the course of its lifetime.

By piecing together the evidence and the origin of M32, the team was able to determine that M32p was the Milky Way’s long-lost sibling, making it the third largest in the ‘Local Group’ after the Andromeda and the Milky Way galaxies. This helps us to understand how disc galaxies evolve and survive large mergers.

After running computer simulations, the scientists were able to understand that even though many companion galaxies were consumed by Andromeda, most of the stars in Andromeda’s outer faint halo were contributed by shredding a single large galaxy.

Leftover weirdo

“Astronomers have been studying the Local Group – the Milky Way, Andromeda and their companions – for so long,” said co-author of the study, Eric Bell.

“It was shocking to realise that the Milky Way had a large sibling, and we never knew about it.”

But it doesn’t end there, as the team suggests that the remaining M32 is the indestructible centre of the original sibling galaxy.

“M32 is a weirdo,” Bell added. “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. There isn’t another galaxy like it.”

The findings are also expected to alter our understanding of how galaxies evolve, as previous theories suggested the collision of two galaxies of this size would only result in the destruction of the discs and the formation of an elliptical galaxy.

The timing of the merger may also explain the thickening of the disc of the Andromeda galaxy as well as a burst of star formation 2bn years ago, a finding that was independently reached by French researchers earlier this year.

Colm Gorey was a senior journalist with Silicon Republic