Black holes: not where they ought to be

By Michael Banks in Miami, Florida

The common view of black holes residing at the centre of their host galaxies might not be completely true, according to astronomer Daniel Bacheldof, from the Florida Institute of Technology.

Speaking today at the 216th American Astronomical Meeting in Miami, Florida, Bacheldof and colleagues used old data taken from the Hubble Space Telescope to show that the supermassive black hole at the centre of the M87 galaxy is slightly displaced from its centre.

The fact that a supermassive black hole – black holes that are millions or billions time the mass of the Sun – can be displaced from the centre of a galaxy is not new, but the fact that astronomers have spotted such a small displacement means that small off-sets could be more common than previously thought.

The explanation for the displacement comes from the fact that the supermassive black hole in M87 was a merger between two smaller black holes. When they merged, the emission of gravitational waves “kicked” the newly created black hole, knocking it slightly off-centre. “What we are seeing in M87 is in effect indirect evidence for gravitational waves,” notes Bacheldof.

The fact that many other supermassive black holes show similar properties to M87 could indicate that such off-sets are common in the universe. “No longer can it be assumed that all supermassive black holes reside at the centres of their host galaxies,” says Bacheldof, who is looking at other such systems to spot similar effects. Time to re-write those astronomy textbooks?