At the centre of our Galaxy lives a monster. A giant black hole four million times the mass of our Sun is lurking, eagerly awaiting the next star or gas cloud to wander into its clutches.
A supermassive black hole like this one is thought to reside in the centre of all galaxies, and astronomers have now discovered a galaxy with three such monsters co-existing in extremely tight quarters.
The galaxy housing the trio was discovered via the Sloan Digital Sky Survey (SDSS), and is given the name SDSS J1502+1115. The galaxy is a quasar, which is the name given to a type of active galaxy that emits enormous amounts of radiation. This radiation emanates from the accretion disc that serves as the food source for the central black holes.
Quasars are plentiful in the early Universe, and indeed this galaxy is located four billion light years from Earth, meaning that the light we see today was actually emitted four billion years ago. Most modern day galaxies lack the fuel to power the quasars, so the central black holes lie dormant.
The trio of supermassive black holes orbit each other more tightly than any other triple system known. The inner pair also have the second tightest orbit known among double black holes, and are separated by only 500 light years. Theoretical models suggest that the inner pair will collide between 10 and 100 million years from now.
It is a cosmic collision that is responsible for placing three supermassive black holes so close together in the first place. Galaxies often collide and merge, so that a once independent central black hole now has to put up with a new neighbour.
The inner pair of black holes were discovered by a technique known as Very Long Baseline Interferometry (VLBI). Signals from radio telescopes on four different continents were combined to produce an image with details 50 times sharper than is possible with the Hubble Space Telescope. The third black hole was discovered from a spectrum, which revealed a pattern of elements that is characteristic of the accretion disc around a supermassive black hole.
These monstrous black holes at the centre of a galaxy can leave quite a trail of destruction. “As these black holes spiral towards one another towards the centre of the galaxy, they might kick up dust, gas and stars,” explains Hans-Rainer Klöckner from the Max Planck Institute for Radio Astronomy. “This leads to increased star formation and black hole accretion, and very possibly the ejection of stars and one of the black holes from the galaxy itself.”
The powerful jets of radiation emitted from the accretion discs of the inner pair are warped by the orbital motion of the black holes. Instead of beacons shining straight into space, the jets twist around each other in a corkscrew shape. The existence of the corkscrew jets for this type of tight binary offers hope of detecting others like it, even in situations where the two cannot be resolved by other methods.
Finding such a collection of supermassive black holes means that they are much more common than previously thought. Roger Deane from the University of Cape Town said: “Because we discovered the inner pair after searching in a small number of galaxies, it suggests that we either won the lottery or that these very close-pair black holes are relatively common. We think the latter.”