Three supermassive black holes discovered in one galaxy
Image: Roger Deane (large image)/NASA Goddard (small image)/Science

Many billions of years ago three galaxies smashed together to form one massive galaxy containing a trio of supermassive black holes.

Two of these black holes orbit each other at 300 times the speed of sound on Earth.

Most galaxies harbour black holes at their centre, objects millions of billions of times heavier than our sun that are so dense that not even light can escape their gravitational pull. 

Astronomer Roger Deane of the University of Cape Town in South Africa and his colleagues who discovered the galaxy, fondly named SDSS J1502+1115, with three black holes reported their finding in Nature this week.

The galaxy is four billion light years away from Earth and is the tightest configuration of black holes ever found (the closest pair is 2.4 kiloparsecs apart). It is one of only four other galaxies known to contain a trio of black holes. 

This discovery opens up further study to scientists hunting for "ripples in spacetime" called gravitational waves. Einstein's theory of relativity predicts that closely orbiting black holes will produce these waves but scientists have yet to find evidence of them in the Universe. 

"As we found this tight pair after searching only six galaxies, we conclude that tight pairs are more common than hitherto believed, which is an important observational constraint for low-frequency gravitational wave experiments," the team reported in their paper.