Galaxies have a trait that is envied by foodies everywhere: They can gobble up other stars, matter and galaxies without increasing their waistline. Any swallowed stars just merge with the galaxy. However, astronomers have spotted one nearby giant galaxy that doesn't seem to have this constant size. It is continuing to expand as it gorges itself on galactic matter.

A team of astronomers, led by Ph.D. student Alessia Longobardi from the Max-Planck-Institut für extraterrestrische Physik (MPE), has been observing this greedy galaxy that's situated 50 million light-years away. The team used ingenious observational techniques to deduce that the galaxy in question, Messier 87, has merged with a spiral galaxy within the last billion years. This intriguing finding has been published in Astronomy & Astrophysical Letters.

"This result shows directly that large, luminous structures in the Universe are still growing in a substantial way – galaxies are not finished yet!" says Longobardi. "A large sector of Messier 87's outer halo now appears twice as bright as it would if the collision had not taken place."

The astronomers didn't come to this conclusion by tracking stars within the galaxy. That would be far too difficult since there are billions of them swirling around in Messier 87. Instead, they looked at planetary nebulae because they give off a distinct aquamarine-green glow that is easy to track amongst the surrounding stars. The name is actually a misnomer as planetary nebulae have nothing to do with planets – they are actually the outer layers of matter that are ejected when a star dies and becomes a white dwarf. 

^{Example of a planetary nebula, NGC 2392, The Eskimo Nebula. NASA, ESA, Andrew Fruchter (STScl), ERO team.}

Using the motion of these beacons to find the origins of their host, Messier 87, is like watching the movement of leaves on a pond to figure out where the ripples in the water originated. The motions the team observed imply that Messier 87 merged with a spiral galaxy. The team also looked at the distribution of light around the galaxy and noticed that the stars that were giving out an excess of light were the ones that had been bumped into during the merger. The extra energy from the disruption caused them to light up more brightly than expected. This merger is also responsible for causing Messier to balloon up to an alarming size.

"We are witnessing a single recent accretion event where a medium-sized galaxy fell through the centre of Messier 87, and as a consequence of the enormous gravitational tidal forces, its stars are now scattered over a region that is 100 times larger than the original galaxy!" commented co-author Ortwin Gerhard of MPE.

[Via European Southern Observatory, Astronomy & Astrophysical Letters]