Most of the galaxies we observe in the universe with our instruments today have a chequered history. Collisions in space in which two galaxies combined occurred over and over again. What happens during this sort of encounter is still not entirely clear. Astronomers are puzzled mainly by the processes in the centres of galaxies where the super-massive black holes are situated. The questions include whether the two super-massive black holes can approach close enough together to form a binary system, and whether they can even merge completely at a later stage. The periods of time during which a transformation of this kind takes place are also unclear.

A realistic model

By using numerical simulations, a research group led by Lucio Mayer, holder of an SNF Professorship at the Institute of Astronomy of ETH Zurich and at the Institute for Theoretical Physics of the University of Zurich, have now found answers to these questions. As published in the scientific journal Science Express (1) in the week beginning 4 June 2007, the researchers calculated very accurately what happens in the interior of the galaxies. Mayer explains that, "The previous models were not sufficiently meaningful because they did not simulate the environment of the super-massive black holes well enough. In contrast, our model takes account of the fact that in addition to stars, the galaxies also have gas and this is non-uniformly distributed in space." The model can also depict processes across several scales of magnitude, which was the only way to treat the events in the centre sufficiently precisely.

The simulations now show that a binary system of two super-massive black holes really is formed inside two combined galaxies, and this occurs in a much shorter time than previously thought. "A twin black hole of this kind is formed within a few hundred thousand years after the galaxies merge." Mayer explains: "That is about 100 times quicker than previously thought." The decisive aspect here has turned out to be the gas which is situated mainly in the centre of the galaxies. The fact that the galaxies combine at all and that the black holes come close together is attributable to frictional forces caused by the stars and the gas. In Mayer's opinion: "Our model shows that the gas contributes the majority of this braking, even when the galaxies have only a comparatively small amount of gas. Considering that galaxies had much more gas in the past than nowadays, the convergence must have happened even faster then."

The two galaxies approach one another over a period of several billion years. The two lowermost images show a section from the centre of the collision. The final phase of the merger to form a binary system from two super-massive black holes takes only a few hundred thousands of years. large

Sources of gravitational waves

If the calculations by Mayer’s group are correct, there must be hundreds of such binary systems in the universe. This is important not only for an understanding of how galaxies combine; according to the General Relativity Theory such binary black holes emit gravitational waves. If there are many such objects in the universe, this increases the chances of detecting their gravitational waves.

Mayer explains, "This is exactly what no-one has succeeded in doing up to now. It has not yet been possible to confirm this aspect of the Theory of Relativity because detecting gravitational waves is an extremely difficult task." That should change now: if everything goes according to plan, NASA and ESA will launch what is known as the "Laser Interferometer Space Antenna" (LISA) (2) into space in the next few years. The costly instrument consists of three freely floating measurement targets whose position is monitored continuously by laser beams. If the predictions of the Relativity Theory are correct, the three targets will be displaced from their position very slightly every time a gravitational wave strikes them. Mayer says, "LISA is one of the most complex measuring instruments ever built by humans. However it is still not yet clear whether the detection of gravitational waves is possible at all." If the project succeeds, it would be an enormous step forwards for physics.