Runaway galaxies ejected by galactic collisions

A recent survey identified 11 isolated, small compact elliptical galaxies.
By Andrew McDonald | Apr 23, 2015
Past discoveries have revealed the existence of around two dozen runaway stars, rocketing out of their former homes at escape velocities of over a million miles per hour. These vagrant stars probably were once members of binary star systems that strayed too close to the supermassive black hole at the heart of the Milky Way; one of the pair is trapped by the black hole's gravity, while the other star is slingshotted away.

Now, astronomers have discovered an entire runaway galaxy. The team, led by Igor Chilingarian of the Harvard-Smithsonian Center for Astrophysics (CfA) and Moscow State University, were using data gathered by the Sloan Digital Sky Survey and GALEX satellite to identify previously unknown compact elliptical galaxies, agglomerations of stars a few hundred light-years across - bigger than star clusters but smaller than other types of galaxies.

According to a CfA statement, the team's survey added nearly 200 to the previous count of 30 known compact ellipticals. However, the most unexpected finding was the existence of 11 compact ellipticals far removed from any bigger galaxy or galactic cluster, in which compact ellipticals usually reside.

These isolated compact ellipticals probably originated through interactions with bigger galaxies. If a compact elliptical were gravitationally paired with a larger galaxy that had siphoned off most of its stars, and that pair interacts with a third galaxy, the compact elliptical could be thrown out. Such wandering compact ellipticals had to have reached an escape velocity of up to six million miles per hour.

The new findings have been published in the journal Science.

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