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Monster galaxies gain weight by eating smaller neighbours

Last Updated on Friday, 19 September 2014 10:57
Published on Friday, 19 September 2014 05:00

Massive galaxies in the Universe have stopped making their own stars and are instead snacking on nearby galaxies, according to research by Australian scientists. They publish their results in the journal Monthly Notices of the Royal Astronomical Society.

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. This simulation shows what will happen when the two galaxies get closer together and then collide, and then finally come together once more to merge into an even bigger galaxy. Simulation 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.

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," he said.

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

Merger-Montage-smallSome of the many thousands of merging galaxies identified within the GAMA survey. Credit: Professor Simon Driver and Dr Aaron Robotham, ICRAR. Click for a full resolution imageDr 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."

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 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," 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 explained.

 

Media contact

Kirsten Gottschalk
Media Contact, ICRAR (Perth, GMT +8:00)
Tel: +61 8 6488 7771
Mob: +61 438 361 876
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University of Western Australia Media Office
Tel: +61 8 6488 7977

 

Science contacts

Dr Aaron Robotham
ICRAR – UWA (Currently travelling in South Africa, GMT +2:00)
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Professor Simon Driver
Principal Investigator of the GAMA project
ICRAR – UWA (Perth, GMT +8:00)
Tel: +61 8 6488 7747
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Images, video and captions

An image of some of the many thousands of merging galaxies identified within the GAMA survey. Credit: Professor Simon Driver and Dr Aaron Robotham, ICRAR

A video of a simulation that shows our galaxy, the Milky Way, merging with the nearby Andromeda 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.

All the related images and video are also available from the ICRAR page on the result.

 

Further Information

The new work appears in Robotham et al., "Galaxy and Mass Assembly (GAMA): Galaxy close-pairs, mergers and the future fate of stellar mass", Monthly Notices of the Royal Astronomical Society, vol. 444 pp. 3986-4008, 2014, published by Oxford University Press. A preprint of the paper is available on the arXiv.

ICRAR is a joint venture between Curtin University and The University of Western Australia with support and funding from the State Government of Western Australia.

 

Notes for editors

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