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Caught in the act: Andromeda Galaxy and neighbouring Triangulum Galaxy hook up



3 September 2009

Caught on film for the first time ever, paparazzi to the stars - astronomers - have captured images of our nearest large galaxy, Andromeda, hooking up with its neighbour, the Triangulum Galaxy. Just like any good star gossip, the liaison was suspected previously, but these images are the first to reveal the connection between the two.

The two star-crossed galaxies: location of the Andromeda Galaxy (Messier 31 or M31) and Triangulum Galaxy (M33) on the sky, with bright stars and constellations marked. These galaxies are named after the constellations in which they are seen.
The two star-crossed galaxies: location of the Andromeda Galaxy (Messier 31 or M31) and Triangulum Galaxy (M33) on the sky, with bright stars and constellations marked. These galaxies are named after the constellations in which they are seen.

Published in the journal Nature on 3 September 2009, the research shows how large galaxies grow by incorporating stars from surrounding smaller galaxies. This popular model of galaxy evolution, called the 'hierarchical model', predicts that large galaxies such as Andromeda, which can be seen with the naked eye from the northern hemisphere, should be surrounded by relics of smaller galaxies it has connected with.

Now astronomers have the images to go with the hierarchical model - a close liaison between the Andromeda and Triangulum galaxies.

The discovery was made by a team of international astronomers, including Professor Geraint Lewis from the University of Sydney's School of Physics. The team was led by Dr Alan McConnachie, from the National Research Council of Canada's Herzberg Institute of Astrophysics, and included astronomers from universities in Canada, Australia, France, Germany, the UK and the USA.

"The Andromeda Galaxy is our closest giant neighbour, located more than 2.5 million light years from the Milky Way. Our new survey charts an area with a diameter of nearly a million light years, centred around Andromeda - it's the broadest and deepest image of a galaxy ever made," said Professor Geraint Lewis.

"We mapped Andromeda's unexplored outskirts for the first time and found stars and giant structures that are remnants of smaller galaxies, which have been incorporated into Andromeda as part of its ongoing growth," explained Professor Lewis.

"The big surprise in the data was finding that Andromeda is interacting with its neighbour, the Triangulum Galaxy, a galaxy which is also visible in the Northern Hemisphere using a small telescope. Millions of Triangulum's stars have been pulled in by Andromeda as part of the encounter."

Surrounding the Andromeda and Triangulum Galaxies are left over relics from Andromeda's formation, including various streams of stars and very small ('dwarf') galaxies, as discovered by an international team of astronomers including Professor Geraint Lewis from the University of Sydney. Distortion around the disk of Triangulum is evidence that it is strongly interacting with its more massive neighbour Andromeda. The dashed circle around Andromeda marks a diameter of approximately 900 000 light years, while that around Triangulum marks approximately 300 000 light years.
Surrounding the Andromeda and Triangulum Galaxies are left over relics from Andromeda's formation, including various streams of stars and very small ('dwarf') galaxies, as discovered by an international team of astronomers including Professor Geraint Lewis from the University of Sydney. Distortion around the disk of Triangulum is evidence that it is strongly interacting with its more massive neighbour Andromeda. The dashed circle around Andromeda marks a diameter of approximately 900 000 light years, while that around Triangulum marks approximately 300 000 light years.

Just like any good star pairing, the paparazzi are predicting a more solid union between the two will result.

"The two galaxies may eventually merge together entirely," explained Professor Lewis.

In addition to the affair between the two galaxies, the new survey has shown that galaxies may be much larger than previously thought, with their gravitational influence stretching well beyond stars near the centre of the galaxy.

"We've found coherent structures and star formations over the entire survey area, showing that galaxies are much bigger than we originally thought. Andromeda is considered by astronomers to be a typical galaxy, so it's surprising to see how vast it really is. We found loosely bound stars at distances up to a hundred times the radius of the large galaxy's central disk."

The team used the Canada-France-Hawaii Telescope, located on the summit of Mauna Kea on the island of Hawaii, to probe the faint outer reaches of Andromeda with unprecedented sensitivity. Their surprising results, reported in Nature, set the stage for a more detailed reconstruction of the formation of Andromeda, a process that appears to be continuing to this day.

A simulation of how Triangulum will eventually join its massive neighbour Andromeda, contributing to the ongoing formation of the Andromeda Galaxy. The image is a stroboscopic projection of a high resolution numerical simulation of a possible orbit of the Triangulum galaxy around Andromeda, that reproduces much of the observed details of these galaxies.
A simulation of how Triangulum will eventually join its massive neighbour Andromeda, contributing to the ongoing formation of the Andromeda Galaxy. The image is a stroboscopic projection of a high resolution numerical simulation of a possible orbit of the Triangulum galaxy around Andromeda, that reproduces much of the observed details of these galaxies.


Watch a movie of the Andromeda and Triangulum Galaxy affair at: www.nature.com/nature/journal/v461/n7260/extref/nature08327-s2.mov

Read the journal article in Nature at: www.nature.com/nature/journal/v461/n7260/full/nature08327.html


Contact: Katynna Gill

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