Cosmic cannibalising: Images show one galaxy engulfing another

As cosmic events go, this one is hard to beat. Scientists have built up a dramatic time-lapse picture of one galaxy swallowing up another in a cannabilistic act that takes place over a period of three billion years – about as long as it took for slime-like earthlings to evolve into humans.

Astronomers have been able to witness a feature of galaxy evolution that they have long suspected but have been unable to visualise whereby one swirling mass of stars devours another that happens to have come within its gravitational sphere of influence.

A telescopic study of the Andromeda galaxy some 2.3 million light years away, the nearest spiral galaxy to our own Milky Way that can be seen with the naked eye, has exposed the galaxy’s immense gravitational tides that are eating away at the smaller Triangulum galaxy as it slowly orbits its galactic master.

The images captured by the Andromeda Archaeological Survey team, using the Canada-France-Hawaii Telescope on the summit of Mauna Kea in Hawaii, show for the first time the immense tidal forces and interactions that cause one galaxy to slowly swallow the stars and cosmic gases of another.

The findings, published in the journal Nature, support a central hypothesis in cosmology called the “hierarchical model” which predicts that large galaxies should be surrounded by the relics of smaller galaxies that the larger galaxies have either completely digested or have begun to engulf.

“Galaxies like our own Milky Way were not born in their current state and they grow by cannabilising smaller galaxies in their path – this is exactly the same with Andromeda,” said Mike Irwin of Cambridge University’s Institute of Astronomy, a member of the international research team.

“What these images tell us is that even galaxies that look beautiful and symmetrical when looked at through a telescope have structure and interactions that you don’t see,” Dr Irwin said.

“This is a startlingly visual demonstration of the truly vast scale of galaxies. The survey has produced an unrivalled panorama of galaxy structure which reveals that galaxies are the result of an ongoing process of accretion and interaction with their neighbour,” he said.

The results of the study were made possible because of the huge area of sky surveyed by the telescope’s powerful digital camera. The observations covered an area with a diameter equivalent to a million light years, a panorama of space that the scientists said is the broadest and deepest view of a galaxy ever made.

One of the problems of finding the evidence to support the hierarchical model of galaxy evolution is that the “leftovers” of the galactic dinner are too faint to be seen over an area that is hundreds of times larger than the galaxy’s central disc of bright stars and gas.

“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,” said Professor Geraint Lewis of the University of Sydney.

“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,” Professor Lewis said.

It takes about 3 billion years for the two galaxies to complete one round of their cosmic dance but eventually they are expected to merge into one entity. Some of the stars in Andromeda are so far from the galactic core that they could only have formed in another galaxy that had long-since been swallowed up.

Another surprise for the survey team is finding the immense scale of a galaxy’s influence. “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,” Professor Lewis said.

“We found loosely bound stars at distances up to a hundred times the radius of the large galaxy’s central disc,” he said.

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