Massive galaxies have stopped making their own stars and started engulfing little nearby galaxies instead, according to a new survey of 22,000 galaxies. Remnants of cannibalized galaxies can still be seen in our own galaxy, but the Milky Way will ultimately get its comeuppance too: We’ll all be devoured by Andromeda in just 5 billion years or so like some chocolate nougat candy bar.
Smaller galaxies are just very efficient at creating stars from gas, whereas the most massive ones hardly produce any new stars themselves. As galaxies grow, they have more gravity that allows them to easily pull in their neighbors.
"All galaxies start off small and grow by collecting gas and quite efficiently turning it into stars," says Aaron Robotham from the University of Western Australia node of the International Centre for Radio Astronomy Research (ICRAR). "Then every now and then they get completely cannibalized by some much larger galaxy."
Star formation slows down in really massive galaxies because of extreme feedback events in the active galactic nucleus, the bright region at the center of a galaxy. Astronomers aren’t sure about the mechanism, but one likely possibility is that the active galactic nucleus basically cooks the gas, preventing it from cooling down to form stars. Ultimately, gravity will cause all the galaxies in bound groups and clusters to merge into super-giant galaxies.
The team collected almost all of the data using the Anglo-Australian Telescope in New South Wales as part of the 7-year-long Galaxy And Mass Assembly (GAMA) survey led by ICRAR’s Simon Driver. The work was published in Monthly Notices of the Royal Astronomical Society this week.
What’s more, our own Milky Way is at a tipping point and is expected to grow mainly by eating smaller galaxies rather than by collecting gas. "The Milky Way hasn't merged with another large galaxy for a long time but you can still see remnants of all the old galaxies we've cannibalized," Robotham says in a news release. "We're also going to eat two nearby dwarf galaxies, the Large and Small Magellanic Clouds, in about four billion years."
But the Milky Way is eventually going to merge with the nearby Andromeda galaxy in 5 billion years. "Technically, Andromeda will eat us because it's the more massive one," he adds.
This simulation shows what will happen when the Milky Way and Andromeda get closer together, collide, and then finally merge into an even bigger galaxy:
Image: Simon Driver and Aaron Robotham, ICRAR
Video: Chris Power (ICRAR-UWA), Alex Hobbs (ETH Zurich), Justin Reid (University of Surrey), Dave Cole (University of Central Lancashire) and the Theoretical Astrophysics Group at the University of Leicester with video production by Pete Wheeler, ICRAR