An artist’s impression of the jets emerging from a
supermassive black hole at the centre of a galaxy.
 Image: Dana Berry / STScI
Supermassive black holes located in the hearts of galaxies are spinning faster than they ever have before, perhaps as a result of recent merging events, report Alejo Martinez-Sansigre of the University of Portsmouth and Steve Rawlings of the University of Oxford.

Although invisible, black holes give themselves away by the churning disc of material that swirls around them before being swallowed, becoming hot and emitting radiation in a final death cry. Twin jets are also often associated with black holes, the spin of which is thought to be important in their generation.

But the evolution of a black hole's spin is a much debated topic. Until recently, it was generally assumed that a black hole would always accrete matter in the same plane of angular momentum. "In this case, provided it grew by approximately 1-3 times its own mass, it would always finish as a maximally spinning black hole," explains Martinez-Sansigre, who adds that a black hole's maximum spin is predicated from theory, based on its angular momentum and mass. "A more recent theory suggests that the material falling into the black hole will come from different directions – chaotic accretion – and approximately half the time the black hole and the matter will end up rotating in the opposite direction, on average resulting in a spin down of the black holes."

Black holes can also grow by merging events, and in this scenario are expected to produce final black holes with high spins. But, adds Martinez-Sansigre, if the black holes were already spinning at their maximum rotation, a merger might lead to a decrease in spin.

Now, by comparing theoretical models of spinning black holes with radio, optical and X-ray observations, Martinez-Sansigre and Rawlings are a step closer to finding out just what causes a black hole's spin to evolve. Their study used the luminosity function of active galactic nuclei – galaxies with signs of non-stellar activity at their centres, attributed to the accretion of matter onto a black hole – with masses a million to a billion times that of our own Sun. While the ages of the black holes are uncertain, they have probably been active for 100 million years or more, and could be much older, possibly billions of years old.

To infer the spin they concentrated on the power of the jets, which are thought to be a function of both black hole spin and accretion rate. "The jets contain strong magnetic fields as well as electrons moving close to the speed of light. In the presence of the magnetic fields, these electrons will radiate away synchrotron radiation, which we can see at radio wavelengths," Martinez-Sansigre tells Astronomy Now. "The jets expand and with some assumptions about their size, their ages, and the density of the intergalactic medium, we can use the radio luminosity to estimate the jet power. We also used X-ray and optical data to estimate accretion rates, and together with the estimates of jet power, that allowed us to estimate the spin."

The team found that black holes with high accretion rates typically had low spins, while black holes with low accretion rates had a bimodal distribution. "About three-quarters to two-thirds had low spins, but the remaining one-quarter to one-third had near maximal spins. This suggested to us that accretion is related to low spins, which agrees with the idea of chaotic accretion," continues Martinez-Sansigre. "The only mechanism left to spin up that fraction of low-accretion black holes is merger events between two black holes of similar mass. If they underwent a merger, but afterwards there was little accretion, then they would retain the relatively high spins from the merger."

The results imply that when the Universe was younger, the majority of black holes had high accretion rates and low spins, compared with the present situation where the black holes with low accretion rates – and a fraction of high spins – are more common. "It seems that on average the black holes have spun up, and we think this is due to them undergoing major mergers with black holes of similar mass, which are not followed by an episode of accretion," adds Martinez-Sansigre.

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