Having already discovered nearly two-dozen runaway stars and even one star cluster that had been ejected from its galaxy, researchers have now reportedly located 11 homeless galaxies that were flung from their home clusters due to gravitational turbulence.
According to Discovery News, the galaxies were found by Igor Chilingarian, an astronomer at the Harvard-Smithsonian Center for Astrophysics and Moscow State University, and his fellow astronomers. They were reviewing publicly available data collected by the Sloan Digital Sky Survey and the GALEX satellite for compact elliptical galaxies and came across these galaxies by happenstance.
The loneliest galaxies
“These galaxies are facing a lonely future, exiled from the galaxy clusters they used to live in,” explained Chilingarian lead author of the paper in the journal Science. Ivan Zolotukhin from Moscow State and the L’Institut de Recherche en Astrophysique et Planetologie, is a co-author of that newly-published study.
Chilingarian and Zolotukhin explained that they originally set out to identify new members of a class of galaxies known as compact ellipticals, miniature groups of stars that are larger than star clusters but smaller than typical galaxies. Their search identified nearly 200 previously unknown compact ellipticals, 11 of which were isolated and found far away from any clusters.
The discovery was unexpected, because the previous 30 compact ellipticals that had been found were all located in clusters. Experts had believed that isolated compact galaxies came from larger galaxies that had been stripped of most of their stars during interactions with larger galaxies, and thus they should be found near those larger galaxies. However, not only where these newfound galaxies isolated, they were also found to be moving faster than those in clusters.
So what caused this phenomenon?
Chilingarian explained that “a classic three-body interaction” was responsible. A star can reach hypervelocity if a binary star system comes too close to a black hole, causing one star to be captured and the other ejected. Similarly, a compact elliptical could be paired with a big galaxy that strips its stars before a third galaxy enters the scene and is accreted by the big galaxy, causing the compact elliptical to be ejected in the process.
Chilingarian and Zolotukhin explained that an object is considered to be a runaway if it travels faster than escape velocity, meaning it will never return to its place of origin. For a runaway star, the required speed is more than one million miles per hour (500 km/s), but for a galaxy, it has to be moving far more quickly, reaching speeds of up to six 6 million miles per hour (3,000 km/s).