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Universe a bit safer than we thought

Colliding galaxies

Two spiral galaxies collide, bringing two black holes together (Image: NASA and Hubble Heritage Team (StscI))

The Universe is not as violent a place as we thought, an Australian astronomer has found.

Dr Alister Graham from the Research School of Astronomy and Astrophysics at the Australian National University in Canberra has published his research in the latest edition of Astrophysical Journal Letters.

"Galactically speaking, things seem to be a little bit safer out there," said Graham, whose research supports the idea that the number of violent encounters between large galaxies is around one-tenth the number suggested by earlier studies.

When two galaxies collide, the supermassive black holes (black holes 10,000 to one million times the mass of the Sun) at their centre go into a binary orbit around a central gravitational well. As this orbit decays, the black holes get closer and their combined massive gravitational pull strips the mass from nearby stars.

Stars continue to be eaten up until the amount of mass stripped from them is equal to or bigger than the combined mass of the black holes.

In the past, direct observations of galaxies suggested that there would have to have been multiple collisions to get enough mass to produce the size of the black holes seen at the centre of galaxies.

But, said Graham, researchers had only been looking at galaxies which were the product of collision.

"People had never carefully looked at the galaxies that hadn't been disrupted and had their centres stripped away," said Graham.

Using the Hubble Space Telescope, Graham studied a sample of galaxies 100 million light years away. He compared the mass of black holes at the centre of collided galaxies, with the amount of star material available in galaxies that were not the product of collisions.

He found that there was enough mass in the stars of an ordinary galaxy to produce a supermassive black hole in as little as just one collision with another galaxy.

"In the past we thought we needed about 10 collisions to cause the damage we thought we were observing around black holes," he said.

Graham's research provides the first direct evidence to support a theory that astronomers have been using for over a decade in their models of the Universe.

But before this, their observations didn't fit the theory.

It gives astronomers more confidence that their modelling of the Universe is "going down the right track", said Graham, who did his research while at the University of Florida.

Tags: science-and-technology, astronomy-space