Astronomers have long suspected that Andromeda is a space predator, consuming dwarf galaxies that wander too close.
Now, cosmic detectives are doing a massive search of the neighbourhood and have found proof of Andromeda's sordid past: They have spotted leftovers in Andromeda's wake.
Early results of a massive telescope scan of Andromeda and its surroundings found about a half-dozen remnants of Andromeda's galactic appetite. Stars and dwarf galaxies that got too close to Andromeda were ripped from their usual surroundings.
"What we're seeing right now are the signs of cannibalism," said the study's lead author Alan McConnachie of the Herzberg Institute of Astrophysics in Victoria, British Columbia. "We're finding things that have been destroyed; ... partly digested remains."
Their report is published in today's edition of the journal Nature.
Andromeda and the Milky Way, Earth's galaxy, are the two big dogs of this galactic neighbourhood. Andromeda is the closest major galaxy, about 2.5 million light years away. A light year is about 5.9 trillion miles.
Astronomers have known for decades that galaxies consume each other, sometimes violently, sometimes just creating new mega-galaxies. But this study is different because "of the scale of the cannibalism, and we've found evidence directly in front of our eyes," said co-author Mike Irwin, an astrophysicist at the University of Cambridge in England.
This type of galactic crash is common, and the paper makes sense, said Harvard astronomer Mark Reid, who was not part of the Andromeda mapping team. And just because Andromeda consumes a galaxy, it does not make the victim disappear, he said.
The cannibalistic behaviour often just strips stars from where they had been, rearranging the night sky. Most of a galaxy is empty space, so there is little if any crashing of stars and planets going on, Irwin said.
"It would be a beautiful night sky," he said. "It would be quite spectacular."
The once and future main victim of Andromeda is a dwarf galaxy that circles it called Triangulum.
Eventually, in about 3 billion years, Triangulam, which once came too close to Andromeda and was stripped of some stars, will spiral into Andromeda, about the same time it comes crashing into Earth's galaxy, said study co-author John Dubinksi of the University of Toronto.
The Milky Way and Andromeda are heading toward each other at about 75 miles per second. They are so far away from each other that the big crash is a few billion years away. And even that might be nothing more than a reshuffling of the night sky or the creation of one super-sized galaxy, McConnachie said.
