11 runaway galaxies ejected from their galaxy clusters found

Scheme of a three-body interaction in which a galaxy is turned into a compat elliptical then is ejected from a cluster (Image NASA, ESA, and the Hubble Heritage Team)
Scheme of a three-body interaction in which a galaxy is turned into a compat elliptical then is ejected from a cluster (Image NASA, ESA, and the Hubble Heritage Team)

There are several known cases of runaway stars such as Zeta Ophiuchi but now the Russian astronomers Igor Chilingarian and Ivan Zolotukhin of the Moscow State University’s Sternberg Astronomical Institute have found runaway galaxies. In an article published in the journal “Science”, the list 11 galaxies ejected from the clusters they used to be part of because of the gravitational interaction with their neighbors.

Igor Chilingarian is working with the Harvard-Smithsonian Center for Astrophysics while Ivan Zolotukhin is working with the Institut de Recherche en Astrophysique et Planétologie of Toulouse, France. Together, they started a study with the intention to identify new members of a class of galaxies called compact elliptical but their research led them to surprising results.

Compact elliptical galaxies are larger than star clusters but smaller than a typical galaxy, having a diameter of a few hundred light years. For a comparison, the Milky Way has a diameter of about 100,000 light years and is not a particularly large galaxy.

Prior to this research, only about 30 compact elliptical galaxies were known and all were part of galactic clusters. To find more, Igor Chilingarian and Ivan Zolotukhin examined data collected from two previous surveys, the SDSS (Sloan Digital Sky Survey) and the GALEX (GALEX), and publicly released inside of the Virtual Observatory initiative. This allowed them to find almost 200 previously unknown compact elliptical galaxies. Of these, 11 were completely isolated and away from any galaxy or galaxy cluster.

This discovery was surprising because according to the theory that type of galaxy is formed from a larger one which is stripped of most of its stars due to interactions with even larger galaxies. For this reason, they’re expect to be found them near large galaxies.

Another surprising element of these isolated compact elliptical galaxies was their very high speed compared to those in the clusters. The explanation given is that were ejected from a cluster as a result of a three-body interaction. Put simply, a galaxy gets stripped of most of its stars by a larger one. Subsequently a third galaxy creates with its gravity a slingshot effect that ejects the compact elliptical galaxy from the cluster.

The escape velocity required to become a runaway galaxy is huge, so much that it can reach 3000 km/s (about 6 million miles per hour). These runaway galaxies remain isolated but for this reason they can survive. In fact, if they remained within a cluster probably they’d be eventually absorbed by one of the other galaxies.

This discovery may be useful to better understand the structure and evolution of compact elliptical galaxies. It’s also a success for the Virtual Observatory initiative, which allows anyone to access the data made public to analyze them in different ways to discover something new.

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