Using the European Southern Observatory’s telescope, astronomers have found, for the first time, evidence of one galaxy existing inside of another galaxy’s stars.
Galaxies are thought to be able to grow by swallowing smaller galaxies. But evidence of this galactic cannibalism is typically difficult to observe. Stars of an infalling galaxy intermingle with similar stars of the bigger galaxy, leaving no trace.
Max-Planck-Institut PhD student Alessia Longobardi lead the team that applied a smart observational trick to clearly show that the nearby giant elliptical galaxy Messier 87 merged with a smaller spiral galaxy in the last billion years.
“This result shows directly that large, luminous structures in the Universe are still growing in a substantial way — galaxies are not finished yet!” says Alessia 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.”
At the centre of the Virgo Cluster of galaxies, sits Messier 87, a vast ball of stars with a total mass more than a million million times that of the Sun, lying about 50 million light-years away.
Instead of attempting to look at all the stars in Messier 87 (there are literally billions, too faint and numerous to be studied individually), the team looked at planetary nebulae, the glowing shells around ageing stars.
Since 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 planetary nebulae are still very faint and need the full power of the Very Large Telescope to study them: the light emitted by a typical planetary nebula in the halo of the Messier 87 galaxy is equivalent to two 60-watt light bulbs on Venus as seen from Earth.
“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. 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,” concludes co-author Magda Arnaboldi
“The build-up of the cD halo of M87 — evidence for accretion in the last Gyr“, by A. Longobardi et al
Photo: Night sky over ESO’s Very Large Telescope (VLT) observatory at Paranal. Credit: ESO/Y. Beletsky