Cosmic threesomes make some galaxies run away

Galactic runaways called compact ellipticals may have been booted out of their homes by a stellar intruder. These small, dense galaxies are normally found cozied up to massive host galaxies that cannibalize their petite neighbors over billions of years. But a couple of strays have stumped astronomers, appearing millions of light-years away from any moochers.

Igor Chilingarian and Ivan Zolotukhin of Moscow State University mined mountains of public astronomy data to show how a third galaxy could bump the small one out of the picture, making it an orphan. The researchers estimate that each compact elliptical, over the course of its lifetime as a galactic snack, will encounter three or four stellar intruders. That provides plenty of opportunity for a compact elliptical to then be knocked out of a host’s orbit, the researchers report in the April 24 Science.

The researchers pinpointed galaxies that showed signs of being previously gobbled, emitted little light and were far enough away to be orphans. The technique yielded 11 such runaways and added 184 compact ellipticals attached to host galaxies to the roughly 30 previously known. The team thinks this same phenomenon could explain the why some rare isolated dwarf galaxies also appear so far from any neighbors.

Editor’s note: This article was updated on April 24, 2015, to clarify that the image is a photo composite/illustration.