Dr Igor Chilingarian of Harvard-Smithsonian Center for Astrophysics and his colleague, Dr Ivan Zolotukhin of the L’Institut de Recherche en Astrophysique et Planetologie in Toulouse, France, have discovered eleven runaway compact elliptical galaxies.
This image illustrates the creation of a runaway galaxy. In the first panel, an ‘intruder’ spiral galaxy approaches a galaxy cluster center, where a compact elliptical galaxy already revolves around a massive central elliptical galaxy. In the second panel, a close encounter occurs and the compact elliptical receives a gravitational kick from the intruder. In the third panel, the compact elliptical escapes the galaxy cluster while the intruder is devoured by the giant elliptical galaxy in the cluster center. Image credit: NASA / ESA / Hubble Heritage Team.
The astronomers initially set out to identify new members of a class of galaxies called compact ellipticals. These objects are bigger than star clusters but smaller than a normal galaxy, spanning a few hundred light-years.
Prior to this study, only about 30 compact elliptical galaxies were known, all of them residing in galaxy clusters.
Dr Chilingarian and Dr Zolotukhin used data from the Sloan Digital Sky Survey and NASA’s GALEX satellite to identify 195 previously unknown compact ellipticals. Of those, 11 were completely isolated and found far from any large galaxy or galaxy cluster.
These isolated compact galaxies were unexpected because theorists thought they originated from larger galaxies that had been stripped of most of their stars through interactions with an even bigger galaxy. So, the compact galaxies should all be found near big galaxies.
Not only were the newfound galaxies isolated, but also they were moving faster than compact ellipticals in clusters.
“We asked ourselves, what else could explain them? The answer was a classic three-body interaction,” said Dr Chilingarian, who is the first author on the study appearing in the journal Science.
A runaway star can be created if a binary system wanders close to a black hole at the core of a massive galaxy. One star gets captured while the other is thrown away at enormous speed.
Similarly, a compact elliptical galaxy could be paired with the big galaxy that stripped it of its stars. Then a third galaxy blunders into the dance and flings the compact elliptical away.
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Igor Chilingarian & Ivan Zolotukhin. 2015. Isolated compact elliptical galaxies: Stellar systems that ran away. Science, vol. 348, no. 6233, pp. 418-421; doi: 10.1126/science.aaa3344
