Astronomers Find Recoiling Supermassive Black Hole

In a Hubble picture, a red circle indicates an object in a distant galaxy that could be an ejected black hole. (Credit: Image courtesy of SRON Netherlands Institute for Space Research)

Astronomers have found a possible supermassive black hole that is recoiling out of a distant galaxy at high speed. The black hole, visible with X-rays as a clear star, is not located in the center of the galaxy, as would normally be the case. Recoiling black holes are interesting because they provide insights into how supermassive black holes develop in the center of galaxies.

Utrecht University student Marianne Heida discovered the bizarre star during her final undergraduate project, undertaken at SRON Netherlands Institute for Space Research, in a galaxy more than half a billion light years away. To make the discovery, she had to compare hundreds of thousands of X-ray sources, picked up by chance, with the positions of millions of galaxies. Normally each galaxy contains a supermassive black hole at its centre that sometimes lights up under X-rays. Yet the star Heida discovered was clearly not located in the center of the system. However, under X-rays the object is so bright that it can best be compared to other bright supermassive black holes in the universe.

A supermassive black hole in the center of a galaxy easily weighs more than 1 billion times the mass of the sun. Such a heavy object could be located so far from the center of a system if it recoils out of the center at considerable speed. The expulsion can take place under special conditions when two black holes merge. The newly formed black hole created after the merging process is then shot out of the center of the system at high speed. Over the last few years various predictions have been made about the speed at which the hole would be slung away. These calculations have only recently become possible, as they require extremely powerful computers. The calculations reveal that the speed of the hole mainly depends on the direction and speed with which the two black holes rotate around their axes before merging.

Heida's research result is probably the tip of the iceberg. Heida says: "We have found even more of this strange class of X-ray sources. However, for these objects we first of all need accurate measurements from NASA's Chandra satellite to pinpoint them more precisely." Finding more recoiling black holes will provide a better understanding of the characteristics of black holes before they merge.

In future, it might even be possible to observe this process with the planned LISA satellite. Astronomers hope to use this satellite to measure the gravity waves that the two merging black holes emit. Ultimately this information must tell us if supermassive black holes in the cores of galaxies are the result of many lighter black holes merging.

Marianne Heida carried out her research at SRON Netherlands Institute for Space Research in Utrecht under the supervision of Peter Jonker. The research results have been accepted for publication in The Monthly Notices of the Royal Astronomical Society. The authors are: Peter G. Jonker (SRON), Manuel A.P. Torres (Harvard-Smithsonian Center for Astrophysics), Andy C. Fabian (Cambridge), Marianne Heida (Utrecht), Giovanni Miniutti (Centro de Astrobiologia), Dave Pooley (Wisconsin).