Tiny and Speedy: 'Homeless' Galaxies Ejected From Clusters

An artwork demonstrates several stages of the process of a close three-body encounter with a gravitation kick of a compact elliptical galaxy that explains the phenomenon of an isolated compact elliptical galaxy.
ESA/Hubble. Artwork by Andrey Zolotov

Excerpt from news.discovery.com

Like stars that can be ejected from galaxies, resigned to an eternity floating through the darkness of intergalactic space, astronomers have discovered entire galaxies — 11 in total — that underwent some unpleasant gravitational turbulence and flung from their home clusters, marooned in intercluster space.

“These galaxies are facing a lonely future, exiled from the galaxy clusters they used to live in,” said Igor Chilingarian, an astronomer at the Harvard-Smithsonian Center for Astrophysics and Moscow State University.

Runaway stars can be ejected from their host galaxies if they are travelling at a greater speed than that galaxy’s “escape velocity.” Like a rocket leaving Earth’s gravitational well, escape velocity can only be achieved if the rocket is supplied with enough energy to exceed 11.2 kilometers per second (25,000 miles per hour). In the case of a star being ejected from our galaxy, it would need to be traveling a speed of 537 km/s (over 1.2 million miles per hour!).

So you can probably imagine the astronomical speed an entire galaxy would need to travel to leave the gravitational heft of an entire galaxy cluster — a velocity of up to 3,000 km/s (6 million miles per hour), depending on the mass of the cluster.

The 11 runaway galaxies were found by chance while Chilingarian and co-investigator Ivan Zolotukhin, of the L’Institut de Recherche en Astrophysique et Planetologie and Moscow State University, were scouring publicly-available data (via the Virtual Observatory) from the Sloan Digital Sky Survey and the GALEX satellite for compact elliptical galaxies.

These tiny galaxies are rare, but the researchers were able to uncover 200 previously unknown compact ellipticals, 11 of which were found to be alone and separated from any galactic cluster. And they are moving really, really fast.

“The first compact ellipticals were all found in clusters because that’s where people were looking. We broadened our search, and found the unexpected,” said Zolotukhin. Elliptical galaxies are thought to originate from larger galaxies that go through gravitational interactions with neighboring galaxies; ellipticals are therefore expected to be clustered near larger ‘parent’ galaxies.

So how did these tiny galaxies, which are approximately 1,000 times smaller than our galaxy, end up so far away from home?
The researchers think that a similar gravitational mechanism that produces runaway stars may be also slingshotting these ellipticals.
“We asked ourselves, what else could explain them? The answer was a classic three-body interaction,” said Chilingarian.

One way a hypervelocity star can be produced is if one star in a binary pair strays too close to a black hole. When the star gets swallowed, its binary partner is flung away. In the case of a hypervelocity compact elliptical galaxy, should a massive galaxy collide with the elliptical’s parent galaxy, the elliptical could be flung away as the two larger galaxies merge.

For the compact elliptical galaxy, this galactic merger is the start of its long and, potentially, infinite journey into the cosmic abyss.