Massive galaxies in the Universe have stopped making their own stars and are instead snacking on nearby galaxies, according to new research.
Astronomers looked at more than 22,000 galaxies and found that while smaller galaxies are very efficient at creating stars from gas, the most massive galaxies are much less efficient at star formation, producing hardly any new stars themselves, and instead grow by eating other galaxies.
Dr Aaron Robotham, who is based at the University of Western Australia node of the International Centre for Radio Astronomy Research (ICRAR), said smaller 'dwarf' galaxies were being eaten by their larger counterparts.
"All galaxies start off small and grow by collecting gas and quite efficiently turning it into stars. Then every now and then they get completely cannibalised by some much larger galaxy," he said.
Robotham, who led the research, said our own Milky Way is at a tipping point and is expected to now 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 cannibalised," he said.
"We're also going to eat two nearby dwarf galaxies, the Large and Small Magellanic Clouds, in about four billion years," he said.
But Robotham said the Milky Way is eventually going to get its comeuppance when it merges with the nearby Andromeda Galaxy in about five billion years.
"Technically, Andromeda will eat us because it's the more massive one," he said.
Almost all of the data for the research was collected with the Anglo-Australian Telescope in New South Wales as part of the Galaxy And Mass Assembly (GAMA) survey, which is led by Professor Simon Driver at ICRAR.
Robotham said as galaxies grow, they have a stronger gravitational field and can therefore more easily pull in their neighbours.
He said the reason star formation slows down in really massive galaxies is thought to be because of extreme feedback events in a very bright region at the centre of a galaxy known as an active galactic nucleus.
"The topic is much debated, but a popular mechanism is where the active galactic nucleus basically cooks the gas and prevents it from cooling down to form stars," Robotham said.
Ultimately, gravity is expected to cause all the galaxies in bound groups and clusters to merge into a few super-giant galaxies, although we will have to wait many billions of years before that happens, researchers said.
The finding was published in the journal Monthly Notices of the Royal Astronomical Society.
