A giant elliptical galaxy called Messier 87 merged with a medium-sized spiral galaxy in the last billion years, according to a comprehensive study of 300 planetary nebulae in the Messier 87 halo observed by ESO’s Very Large Telescope (VLT).
The huge halo around the giant elliptical galaxy Messier 87 appears on this image. The image also reveals many other galaxies forming the Virgo Cluster, of which Messier 87 is the largest member. Image credit: Chris Mihos, Case Western Reserve University / ESO.
“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,” said Alessia Longobardi of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, lead author of the paper in the journal Astronomy & Astrophysics (arXiv.org preprint).
Messier 87, also known as M87, Virgo A or NGC 4486, is located at the center of a galaxy cluster in the constellation of Virgo, about 50 million light-years away.
The galaxy has a total mass more than a million million times that of the Sun and hosts one of the most massive black holes discovered so far, with a mass 6 billion times that of the Sun. Every few minutes this black hole swallows an amount of matter similar to that of the whole Earth, converting part of it into radiation and a larger part into powerful jets of ultra-fast particles.
There are billions of stars in this supergiant galaxy, but they are too faint and numerous to be studied individually. Rather than try to look at the stars, Ms Longobardi and her colleagues looked at planetary nebulae in the halo of Messier 87.
“These planetary nebulae are the only single objects whose motions can be measured at Messier 87’s distance of 50 million light-years from Earth. They behave like beacons of green light and as such they tell us where they are and at what velocity they are traveling.”
“The nebulae are still very faint and need a powerful telescope to study them: the light emitted by a typical planetary nebula in Messier 87 is equivalent to two 60-watt light bulbs on Venus as seen from Earth.”
The motions of 300 planetary nebulae in the Messier 87 halo, measured using ESO’s VLT, provide 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,” explained co-author Dr Ortwin Gerhard, also from the Max Planck Institute for Extraterrestrial Physics.
The astronomers also looked at 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.
These observations have also shown that the disrupted galaxy has added younger, bluer stars to Messier 87, and so it was probably a star-forming spiral galaxy before its merger.
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A. Longobardi et al. 2015. The build-up of the cD halo of M 87: evidence for accretion in the last Gyr. A&A 579, L3; doi: 10.1051/0004-6361/201526282
