Scientists have found a mysterious galaxy, twinned with our own, thought to have been shredded apart.
Even though the galaxy has been mostly destroyed, it left behind an intriguing trail of evidence. The universe is haunted by bits left behind from when it was around, such as an almost invisible halo of stars larger than the Andromeda galaxy itself and an elusive stream of stars, the researchers write.
The galaxy was ripped to shreds by Andromeda, our nearest galactic neighbour, the new paper claims. But it was once the third-largest galaxy in our local area.
Scientists have long known that the halo of stars contained the remnants of cannibalised galaxies. But they feared that there was so many smaller galaxies contained within that it would be impossible to learn about any specific one.
New computer simulations allowed them to find that while Andromeda had eaten many galaxies, many of the stars found in the halo actually came from one larger shredded galaxy.
"It was a 'eureka' moment. We realised we could use this information of Andromeda's outer stellar halo to infer the properties of the largest of these shredded galaxies," said lead author Richard D'Souza, a postdoctoral researcher at the University of Michigan.
That galaxy is a long-lost sibling of our own.
"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," said co-author Bell, professor of astronomy at the same university.
The researchers hope that the discovery can lead to new breakthroughs in understanding how galaxies form and evolve.
It also explains some of the strange behaviour in our own universe. It could answer the puzzle of why the M32 satellite galaxy near Andromeda has come from – it might be the surviving core of that long-lost sibling, sticking around like the core in a plum.
"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. There isn't another galaxy like it."
