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Bully galaxy rules the neighbourhood
DR EMILY BALDWIN
ASTRONOMY NOW
Posted: 4 March 2010


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Galaxies are often found in groups, interacting with their neighbours, but a recent Hubble Space Telescope image reveals that some galaxies are loners, gobbling up other galaxies that lie in their path.

ESO 306-17 is a galactic bully, but having feasted on galaxies that dare strayed into its path, it now lies alone in space. Image: NASA, ESA and Michael West (ESO).

A large bright elliptical galaxy, ESO 306-17, sets the scene. On first glance it looks like ESO 306-17 is surrounded by galaxies, but these are just foreground galaxies, leaving the giant galaxy abandoned in a sea of dark matter and hot gas. Loner galaxies like this are often known as fossil groups.

Where did its neighbours go? Is ESO 306-17 a remnant of a once active galactic community, or is there something more sinister going on? Scientists speculate that the behemoth could have snacked on its smaller neighbours that strayed into its gravitational clutches. It is well known that galaxies can accumulate mass in this way – indeed there is evidence to suggest that our own Milky Way Galaxy fed on smaller dwarf galaxies to attain its size – but ESO 306-17 and other galactic loners may represent some of the most extreme examples of galaxy cannibals. These systems simply keep on feeding until all of the nearby galaxies have been swallowed up.

Taking a closer look at Hubble's find reveals faint groups of globular clusters, dense star systems that can withstand the 'bully' galaxy, through the bright shine of the galaxy's halo. Studying these surrounding clusters will prove helpful to astronomers in their pursuit to piece together ESO 306-17's history.

Researchers will also this image to hunt for ultra-compact dwarf galaxies, those which have been stripped to their core due to the interaction with larger, more powerful galaxies. The majority of known ultra-compact dwarf galaxies reside near giant elliptical galaxies in large clusters, so it will be interesting to see if researchers find similar objects around isolated fossil groups.

The image was captured with Hubble's Advanced Camera for Surveys in November 2008.

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