Very Large Telescope Spots Galactic Cannibalism: Galaxy Swallows Smaller Cousin

Astronomers have spotted a galaxy that didn't just swallow another galaxy, but is now growing. They've discovered that the nearby giant elliptical galaxy Messier 87 merged with a small spiral galaxy in the last billion years.

Researchers believe that galaxies grow by swallowing smaller galaxies. The evidence of this, though, is not easy to see. The stars in the infalling galaxy merge with the very similar stars of the larger galaxy, leaving no trace.

Messier 87 lies at the center of the Virgo cluster of galaxies. It's a vast ball of stars with a total mass that's more than a million times that of the sun, lying about 50 million light-years away.

In this latest study, the researchers examined planetary nebulae in this galaxy, which are the glowing shells around aged stars. Because these objects shine very brightly in a specific hue of aquamarine green, they can be distinguished from the surrounding stars. Careful observation of the light from the nebulae using a powerful spectrograph can also reveal their motions. These motions, in turn, provided clues to the past merger.

"We are witnessing a single recent accretion event where a medium-sized galaxy fell through the center 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!" said Ortwin Gerhard, co-author of the new study, in a news release.

There researchers also examined the light distribution in the outer parts of Messier 87 and found evidence of extra light coming from the stars in the galaxy that had been pulled in and disrupted. This also showed that the disrupted galaxy has added younger, bluer stars to Messier 87, and so was probably a star-forming spiral galaxy before its merger.

"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-but still to be able to see from their velocities that they belong to a common structure," said Magda Arnaboldi, co-author of the study. "The green planetary nebulae are the needles in a haystack of golden stars. But these rare needles hold the clues to what happened to the stars."

The findings are published in the journal Astronomy & Astrophysics Letters.

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