Supermassive black holes: we’ve spotted signs of mergers that may finally help us prove they exist

Visible light image of the radio galaxy Hercules A obtained by the Hubble Space Telescope superposed with a radio image taken by the Very Large Array of radio telescopes in New Mexico, USA. NASA

Supermassive black holes: we’ve spotted signs of mergers that may finally help us prove they exist

Observations of nature tend to throw up unexpected results and new mysteries – whether you’re investigating the rain forest or outer space. When radio astronomy took off in the 1950s, we had no idea that it would lead to the discovery that galaxies including our own seem to have terrifyingly large black holes at their centre – millions to billions of times the mass of the sun.

A few decades later, we still haven’t been able to prove that these beasts – dubbed supermassive black holes – actually exist. But our new research, published in the Monthly Notices of the Royal Astronomical Society, could one day help us do so.

Early radio astronomers discovered that some galaxies emit radio waves (a type of electromagnetic radiation). They knew that galaxies sometimes collide and merge, and naturally wondered whether this could have something to do with the radio emission. Better observations, however, refuted this idea over the years.

They also discovered that the radio waves were emitted as narrow jets, meaning that the power came from a tiny region in the nucleus. The radio power was indeed huge – often surpassing the luminosity of all the stars in the galaxy taken together. Various suggestions were made as to how such a huge amount of energy could be produced, and it was in the 1970s that scientists finally proposed that a supermassive black hole could be the culprit. The objects are nowadays known as quasars.

Theoretical models estimated that these objects would have a mass of an entire small galaxy concentrated in a space comparable to Earth’s orbit around the sun. But because only some galaxies produce energetic outbursts, it was unclear how common supermassive black holes would be. With the advent of the Hubble Space Telescope in 1990, the centres of nearby galaxies that did not emit radio bursts could finally be investigated. Did they contain supermassive black holes too?

It turned out that many did – astronomers saw signs of gravitating masses influencing the matter around it without emitting any light. Even the Milky Way showed evidence of having a supermassive black hole at the centre, now known as Sgr A*. At this point, astronomers became increasingly convinced that supermassive black holes were a reality and could plausibly explain the extreme energetic outbursts from some galaxies.

However, there is no definitive proof yet. That is despite the fact that some supermassive black holes emit jets – these come from the surroundings of the black hole rather than the black hole itself. So how do you prove the existence of something completely dark? A black hole as defined by Einstein’s theory of general relativity is a region of space bounded by a horizon – a surface from inside of which no light or material object can ever escape. So, it’s a pretty difficult task for astronomers: they need to see something that emits nothing.

Black hole collision and merger releasing gravitational waves. LIGO, CC BY-SA

For smaller black holes the size of a stellar mass, a proof was indeed found: when two such objects merge, they emit gravitational waves, a tiny wobbling of space that was for the first time registered in 2015. The detection proved that black holes exist, that they sometimes form pairs and that they indeed merge. This was a tremendous success, honoured with the Nobel prize in 2017.

We also have a good understanding of where normal sized black holes come from – they are what is left after a star much more massive than the sun has arrived at the end of its lifetime. But both the existence and the origin of supermassive black holes are shrouded in mystery.

Spinning black holes

We have now found indications that many of the radio jets produced by supermassive black holes may in fact be the result of these objects forming pairs, orbiting each other. We did this by comparing the observed radio maps of their regions with our computer models.

The presence of a second black hole would make the jets produced by the first one change direction in a periodic way over hundreds of thousands of years. We realised that the cyclic change in jet direction would cause a very specific appearance in radio maps of the galaxy centre.

Lobes are created by the jets depositing energy to surrounding particles. Author provided

We found evidence of such a pattern in about 75% of our sample of “radio galaxies” (galaxies that emit radio waves), suggesting that supermassive black hole pairs are the rule, not the exception. Such pairs are actually expected to form after galaxies merge. Each galaxy contains a supermassive black hole, and since they are heavier than all the individual stars, they sink to the centre of the newly formed galaxy where they first form a close pair and then merge under emission of gravitational waves.

While our observation provides an important piece of evidence for the existence of pairs of supermassive black holes, it’s not a proof either. What we observe are still the effects that the black holes somehow cause indirectly. Just like with normal black holes, a full proof of the existence of supermassive black hole pairs requires detection of gravitational waves emitted by them.

Current gravitational wave telescopes can only detect gravitational waves from stellar mass black holes. The reason is that they orbit around one another much faster, which leads to the production of higher frequency gravitational waves that we can detect. The next generation of instruments will however be able to register low frequency gravitational waves as well – potentially from supermassive black hole pairs. This would finally prove their existence – half a century after they were first proposed. It’s an exciting time to be a scientist.

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