Astronomers have found 11 runaway galaxies that have been flung out of their home clusters.
The study, published in the journal Science, was conducted by astronomer Igor Chilingarian from the Harvard-Smithsonian Center for Astrophysics/Moscow State University along with Ivan Zolotukhin from L’Institut de Recherche en Astrophysique et Planetologie/Moscow State University.
“These galaxies are facing a lonely future, exiled from the galaxy clusters they used to live in,” said Chilingarian, who is the lead author of the study.
An object is called a runaway if its movement is faster than escape velocity, meaning that it will depart its home, never to return. The speed of a runaway star is more than a million miles per hour, or 500 km/s, and a runaway galaxy may be traveling at up to 6 million miles per hour, or 3,000 km/s, Harvard-Smithsonian Center for Astrophysics said in a statement.
In the beginning, Zolotukhin was identifying new members of a class of galaxies, called compact ellipticals, which weigh 1000 times less than a galaxy like the Milky Way. These tiny spots of stars are bigger than star clusters, however, these are smaller than a typical galaxy. The Milky Way is 100,000 light-years across.
The researchers found that almost 200 compact ellipticals that were previously unknown. A total of 11 were completely isolated and found far from any large galaxy or galaxy cluster. Previously, about 30 compact elliptical galaxies were identified. These are residing in galaxy clusters.
“The first compact ellipticals were all found in clusters because that’s where people were looking. We broadened our search, and found the unexpected,” Zolotukhin said.
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. So, the compact galaxies should all be found near big galaxies. Not only were the newfound compact ellipticals isolated, but also they were moving faster than their brethren in clusters.
“We asked ourselves, what else could explain them? The answer was a classic three-body interaction,” said Chilingarian.
A hypervelocity star can be created if a binary star system wanders close to the black hole at the center of our galaxy. One star gets captured, while the other is thrown away at tremendous speed.
Similarly, a compact elliptical could be paired with the big galaxy that stripped it of its stars. Then, a third galaxy blunders into the dance and flings the compact elliptical away. As punishment, the intruder gets accreted by the remaining big galaxy.
“We recognized we could use the power of the archives to potentially unearth something interesting, and we did,” Chilingarian added.