We have spotted traces of collisions that could ultimately help us prove the existence of supermassive black holes.

Nature's observations tend to generate unexpected results and new mysteries – whether it's investigating the rainforest or space. When radio astronomy took off in the 1950s, we had no idea that it would lead to the discovery that galaxies, including ours, seem to have black holes of a terrifying size in their center. , millions of billions of times the mass of the sun.

A few decades later, we still have not been able to prove that these animals, called supermassive black holes, really exist. But our new research, published in the Monthly Notices of the Royal Astronomical Society, may one day help us do that.

The first radio astronomers discovered that some galaxies emitted radio waves (a type of electromagnetic radiation). They knew that galaxies sometimes met and melted, and naturally wondered if this could have anything to do with the radio show. Better observations, however, have refuted this idea over the years.

They also discovered that radio waves were emitted in the form of narrow jets, which meant that the energy was coming from a small region of the nucleus. The power of the radio was indeed huge – often exceeding the brightness of all the stars in the galaxy taken together. Various suggestions have been made on how to produce such a large amount of energy. It was in the 1970s that scientists finally proposed the creation of a supermassive black hole. Objects are now called quasars.

Theoretical models have estimated that these objects would have the mass of a small whole galaxy concentrated in a space comparable to Earth's orbit around the sun.
But since only certain galaxies produce energetic explosions, it was difficult to know how common supermassive black holes would be. With the advent of the Hubble Space Telescope in 1990, the centers of nearby galaxies that did not radiate a start could finally be explored. Do they also have supermassive black holes?

Many astronomers have found that gravitational masses influence the surrounding matter without emitting light. Even the Milky Way has shown a supermassive black hole in the center, 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 energy explosions of certain galaxies.

However, there is no definitive proof yet. This is despite the fact that some supermassive black holes emit jets – these come from the black hole environment rather than the black hole itself. So, how to prove the existence of something completely black? A black hole as defined by Einstein's theory of general relativity is a region of space bounded by a horizon – a surface from which no light or material object can escape. So it's a pretty difficult task for astronomers: they need to see something that does not emit anything.

For smaller black holes the size of a stellar mass, evidence has been found: when two objects of this type fuse, they emit gravitational waves, a tiny oscillation of the recorded space for the first time in 2015. The detection has proved the existence of black holes. that they sometimes form pairs and that they blend well. It was a huge success, awarded by the Nobel Prize in 2017.

We also have a good understanding of the origin of normal sized black holes – this is what remains after the formation of a much larger star than the sun at the end of its life. But the existence and origin of supermassive black holes are surrounded by mystery.

Swirling black holes

We have now found indications that many of the radio jets produced by supermassive black holes may actually be the result of the formation of pairs of objects in orbit. We did this by comparing the observed radio maps of their regions with our computer models.

The presence of a second black hole would periodically change the direction of the jets produced by the first over hundreds of thousands of years. We realized that the cyclic change in the direction of the jet would cause a very specific appearance in the radio maps of the center of the galaxy.

We have found evidence of such a pattern in about 75% of our sample of "radio galaxies" (galaxies emitting radio waves), suggesting that pairs of supermassive black holes are the rule and not the only one. ;exception. Such couples should actually form after galaxy fusion. Each galaxy contains a supermassive black hole and, since they are heavier than all individual stars, they sink into the center of the newly formed galaxy where they first form a close pair and blend together. then under the emission of gravitational waves.

Our observation provides important evidence for the existence of pairs of supermassive black holes, but it is also not a proof. What we are observing is always the effects that the black holes have indirectly caused. As with normal black holes, a complete proof of the existence of pairs of supermassive black holes requires the detection of the gravitational waves emitted by these pairs.

Current gravitational wave telescopes can only detect gravitational waves from stellar mass black holes. The reason is that they revolve around each other much more quickly, resulting in the production of high-frequency gravitational waves that we can detect. The next generation of instruments will however be able to record low frequency gravitational waves, potentially pairs of supermassive black holes. This would finally prove their existence – half a century after their first proposal. It's an exciting time to be a scientist.