Messier 87 – a supergiant elliptical galaxy some 50 million light years away in the heart of the Virgo Cluster – is still growing, according to new evidence that reveals it has swallowed an entire medium-sized galaxy within the last billion years.
Galaxies are said to grow in size by gobbling up smaller ones. However, evidence that this has happened isn’t always easy to see, since they often leave no trace of their cannibalistic behaviour. Not quick to be put off by a trail that has gone cold, Alessia Longobardi at the Max Planck Institute for Extraterrestrial Physics in Garching in Germany, has applied a clever observational trick to clearly show that Messier 87 merged with a comparatively smaller spiral galaxy many years ago. “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.”
Since Messier 87 is a vast ball of stars with some stellar members being barely visible, Longobardi and her team took to looking at planetary nebulae – the glowing shells found around ancient stars. Since these objects were found to shine brightly in ghostly shades of green, they were able to be distinguished from stars. Not only that, but getting a good look at their light meant that the team were able to work out their motions and therefore trace the history of the merger.
Ortwin Gerhard, head of the dynamics group at the Max Planck Institute for Extraterrestrial Physics, says of the ancient galactic feast: “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!”
Looking carefully at light in the outer parts of the elliptical galaxy, the team found that there seemed to be an additional glow from its stars that had been pulled and distorted. What’s more, the disrupted structure has shared some of its younger, bluer stars with Messier 87, indicating that it was most likely a galaxy brimming with star formation before its fate.
“It is very exciting to be able to identify stars that have been scattered around hundreds of thousands of light years in the halo of this galaxy,” adds the European Southern Observatory (ESO)’s Magda Arnaboldi. “The green planetary nebulae are the needles in a haystack of red stars. But these rare needles hold the clues to what happened to the stars.”