Supermassive black holes collide

If we can't see them, how do we know they're there?
26 October 2018


Astronomers at the University of Hertfordshire have come up with an explanation for the wobble seen in jets of matter being blasted from regions surrounding some supermassive black holes: Another nearby supermassive black hole!

There are actually two types of black hole: stellar mass black holes and supermassive black holes. Lead author, Dr Martin Krause explains the difference: “When a star that’s about 10 times the mass of the sun comes to the end of its life, it collapses and forms these [stellar mass] black holes, 3 to 10 times the mass of the sun. Then in the centres of galaxies we have evidence for a lot of mass that produces a huge amount of energy, so the mass you need is at least a million times the mass of the sun.”

Though we can't observe supermassive holes directly, it is strongly suspected that enormous astrophysical jets that criss-cross the sky are generated by them. These jets are made of elementary particles, mainly electrons, which fly from the black holes at close to the speed of light. These particle jets generate radio waves which can be detected here on Earth with large radio telescopes.

If two supermassive black holes are in fact orbiting one another, then as they spin the jet will be being pulled periodically in different directions, causing a repeated wobble of the direction of the jet. The team created new super computer simulations which included the possibility of these “binary” supermassive black holes, to compare to observational data that had already been examined for over a decade, and discovered a much better fit.

The existence of black holes was proven in 2015 by measuring gravitational waves as two black holes merge, leading to the award 2017 Nobel Prize in Physics for that research. It was this discovery that caused Dr Krause to look at his data in a new way, considering the possibility that two supermassive black holes could be heading towards merging: “I took a sample of jet sources and looked at them from the perspective of how many of them could be explained with a [jet] changing direction. It’s quite obvious that quite a few of them have this procession. We find actually that 75% of them show this.”

By understanding how these jets are formed, we can better understand the history of the supermassive black holes that produce them. This could lead us to making better models of the formation of the galaxies containing central black holes, and how supermassive black holes form.


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