A team of scientists from the Harvard-Smithsonian Center for Astrophysics and Moscow State University have spotted 11 runaway galaxies that have been flung out of their homes to wander the void of intergalactic space. While stars and even star clusters have been seen “escaping” galaxies, observing actual galaxies themselves is a whole other final frontier in astrophysics.
The discovery, which was published in the journal Science cited lead astronomer Igor Chilingarian who said: “These galaxies are facing a lonely future, exiled from the galaxy clusters they used to live in.”
If you wondering how a galaxy can be a “runaway”, you’re not the only, but according to Chilingarin and his team, an object is classed as a runaway if it’s moving faster than escape velocity, which means it can depart its home never to return. For runaway stars, that speed is more than a million miles per hour (500 km/s), but runaway galaxies have to go even faster, traveling at up to 6 million miles per hour (3,000 km/s)—and the team have spotted 11!
The team’s orginal objective was to identify new members of a class of galaxies called compact ellipticals, which are groups of stars that are bigger than star clusters but smaller than a typical galaxy, spanning “only” a few hundred light-years. (For those of you needing some comparison, our galaxy, The Milky Way, is 100,000 light years across).
Only 30 or s0 compact elliptical galaxies were known before the study was undertaken, all of them residing in galaxy clusters. However, Chilingarian and his team identified almost 200 previously unknown compact ellipticals, 11 of which were strangely isolated and far from any large galaxy or galaxy cluster. “The first compact ellipticals were all found in clusters because that’s where people were looking. We broadened our search, and found the unexpected,” co-author Ivan Zolotukhin.
These isolated compact galaxies were unexpected because theorists thought they originated from larger galaxies that had been stripped of most of their stars through interactions with an even bigger galaxy. While a hypervelocity star can be created if a binary star system wanders close to the black hole at the center of our galaxy, a compact elliptical could, in theory, be paired with a big galaxy that’s been stripped it of its stars. If a third galaxy then gets caught up in the gravitational dance, then it can potentially fling the compact elliptical away.
“It’s a classic three-body interaction,” notes Chilingarian.
So there you have it from an astrophysicist: if three bodies are involved, one inevitably gets hurled away!