Monster galaxies gain weight by eating smaller neighbors

September 19, 2014
These are some of the many thousands of merging galaxies identified within the GAMA survey. Credit: Credit: Professor Simon Driver and Dr Aaron Robotham, ICRAR.

Massive galaxies in the Universe have stopped making their own stars and are instead snacking on nearby galaxies, according to research by Australian scientists.

Astronomers looked at more than 22,000 and found that while smaller galaxies are very efficient at creating from gas, the most 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," he said.

"Then every now and then they get completely cannibalised by some much larger galaxy."

The study was released today in the journal Monthly Notices of the Royal Astronomical Society, which is published by Oxford University Press.

Dr 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.

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In about five billion years time, nearby massive galaxy Andromeda will merge with our own galaxy, the Milky Way, in an act of galactic cannibalism (technically Andromeda will be eating us, as it's the bigger of the two galaxies.). There haven't been any large mergers with our galaxy recently, but we can see the remnants of galaxies that have previously been snacked on by the Milky Way. We're also going to eat two nearby dwarf galaxies, the Large and Small Magellanic Clouds sometime in the future. This simulation shows what will happen when the Milky Way and Andromeda get closer together and then collide, and then finally come together once more to merge into an even bigger galaxy. Credit: Prof Chris power (ICRAR-UWA), Dr Alex Hobbs (ETH Zurich), Prof Justin Reid (University of Surrey), Dr Dave Cole (University of Central Lancashire) and the Theoretical Astrophysics Group at the University of Leicester.Video Production Credit: Pete Wheeler, ICRAR.

"We're also going to eat two nearby dwarf galaxies, the Large and Small Magellanic Clouds, in about four billion years."

But Dr 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.

The GAMA survey involves more than 90 scientists and took seven years to complete.

This study is one of over 60 publications to have come from the work, with another 180 currently in progress.

Dr Robotham said as galaxies grow they have more gravity and can therefore more easily pull in their neighbours.

Dr Aaron Robotham from the International Centre for Radio Astronomy Research. Credit: Dr Joe Liske

He said the reason 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 basically cooks the gas and prevents it from cooling down to form stars," Dr 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.

"If you waited a really, really, really long time that would eventually happen but by really long I mean many times the age of the Universe so far," Dr Robotham said.

Explore further: The Milky Way now has a twin (or two)

More information: 'Galaxy and Mass Assembly (GAMA): Galaxy close-pairs, mergers and the future fate of stellar mass' Monthly Notices of the Royal Astronomical Society, 2014..

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Da Schneib
3 / 5 (2) Sep 19, 2014
This is important to our simulations of galactic evolution, and will inform them and make them more accurate (and they're already pretty accurate). Good news, and yet another great result from this research program.

Good article too. :D

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