Messier 87. Red and blue dots pinpoint the planetary nebulae whose motion revealed that the galaxy had been struck by a smaller galaxy. Those marked by red are moving away from us, while those by blue are moving toward us. Credit: A. Longobardi (Max-Planck-Institut für extraterrestrische Physik)/C. Mihos (Case Western Reserve University)/ESO
Giant elliptical galaxy Messier 87 has swallowed a smaller-sized galaxy during the past billion years and consequently shines twice as bright as it otherwise would.
It is thought that the smaller galaxy fell through the centre of Messier 87 and gravitational tides caused it to scatter its stars over a region 100 times larger than the original galaxy.
A team at the Max Planck Institute for Extraterrestrial Physics (MPE) in Munich tracked the motions of 300 glowing planetary nebulae to find evidence of the event.
Messier 87 is in the Virgo cluster and has a total mass over a million million times that of the Sun.
Rather than attempt to study the individual stars of a galaxy that size, the team was able to uncover the galactic cannibalism by looking at planetary nebulae, the glowing shells around ageing stars.
These objects shine bright green, distinguishing them from the surrounding stars, and the team spotted their movement, caused by the merger, by observing the light through the FLAMES spectrograph on the Very Large Telescope.
Like the ripples when a glass of water is thrown into a larger body of water, the motions of these planetary nebulae gave the team evidence that the galactic merger had occurred.
The study also looked at light distribution in the outer edges of Messier 87 and found that extra light was emanating from stars originally contained within the smaller galaxy.
The swallowed galaxy added younger, bluer stars and so was probably a star-forming spiral galaxy before the collision.
PhD student Alessia Longobardi, who led the study at MPE, says:
“This result shows directly that large, luminous structures in the Universe are still growing in a substantial way — galaxies are not finished yet!
A large sector of Messier 87's outer halo now appears twice as bright as it would if the collision had not taken place.”
"It is very exciting to be able to identify stars that have been scattered around hundreds of thousands of lightyears in the halo of this galaxy — but still to be able to see from their velocities that they belong to a common structure," says co-author Magda Arnaboldi from ESO.
"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."