Astronomers discover runaway galaxies

Compact ellipticals found isolated from larger galaxy clusters

The creation of a runaway galaxy
Credit: Harvard-Smithsonian Centre for Astrophysics

Astronomers have discovered 11 ‘runaway’ galaxies that have been exiled from the galaxy clusters in which they used to live.

Astronomy has already noted the existence of runaway stars that have escaped their galaxies, and even a runaway star cluster, but this latest discovery scales up the concept of runaway objects substantially.

An object might be described as a ‘runaway’ if it is moving faster than escape velocity, which is the speed required to break free of the gravitational attraction caused by a large body. When an object breaks this gravitational pull, it is flung into space, away from its galactic home forever.

For runaway stars, the speed required for this process to occur is over 1.6 million kilometres per hour, but a runaway galaxy needs to travel six times that speed.

Igor Chilingarian of the Harvard-Smithsonian Center for Astrophysics and Moscow State University is the lead author of this latest study on runaway objects. He and co-author Ivan Zolotukhin of L'Institut de Recherche en Astrophysique et Planetologie and Moscow State University began the study by attempting to identify types of smaller galaxy called compact ellipticals. These collections of stars are bigger than clusters but smaller than galaxies, spanning just a few hundred lightyears.

Their research led to an analysis of data from the Sloan Digital Sky Survey based in New Mexico and NASA’s Galaxy Evolution Explorer (GALEX) satellite. This analysis uncovered about 200 unknown compact ellipticals, 11 of which were entirely separate and located far from any large galaxy or cluster.

It was thought that these compact ellipticals were originally large galaxies that had been stripped of most of their stars by even bigger galaxies. But the fact that they were found so far from any bigger galaxies seemed to disprove this theory.

"The first compact ellipticals were all found in clusters because that's where people were looking. We broadened our search and found the unexpected," says Zolotukhin.

“We asked ourselves, what else could explain them? The answer was a classic three-body interaction,” says Chilingarian.

The three-body situation to which Chillingarian is referring sees a compact elliptical paired with the bigger galaxy that absorbed its stars. When a third galaxy enters into the mix, it nudges the compact elliptical out of the way, merging together with the bigger galaxy. The compact elliptical is then flung into isolated space.

"We recognised we could use the power of the archives to potentially unearth something interesting, and we did," says Chilingarian says.

Front page image: Abell 1689, a large cluster of galaxies 2.3 billion light years away that shows signs of merging activity
Credit: X-ray: NASA/CXC/MIT/E,-H Peng et al; Optical: NASA/STScI
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