An international team of researchers now seems to have figured it out: it has everything to do with the fact that our telescopes are biased against these massive black holes.

It is 2015 when specially developed detectors on Earth detect a gravitational wave for the first time. In addition to the great joy that it has finally been possible to actually detect the gravitational waves predicted by Einstein, astronomers are also amazed. The wave appears to have been created during a collision between two black holes that belong to a type that we did not know existed until that moment. With a weight of several tens of solar masses, both black holes can be classified as heavy black holes. And astronomers had never observed these with their conventional telescopes (see box).

Black holes are created when stars come to an explosive end. They are quite compact, but have immense gravity. As a result, nothing can escape the grip of a black hole. Not even light. It means that we can’t see black holes either. But conventional telescopes can indirectly observe black holes when they are guided by a star. When a black hole steals matter from that star, it releases electromagnetic radiation, which reveals that there is a black hole near the star. The mass of that black hole can then also be determined on the basis of the orbit of the companion star. In this way, researchers had already spotted quite a few bright black holes prior to 2015. But never before had a massive black hole, as witnessed by gravitational waves, been observed. And in 2021 that has not yet changed; although we have already collected quite a few gravitational waves caused by these massive black holes, the counter of the number of massive black holes observed with conventional telescopes is still at zero.

bias

How is that possible? An international team of researchers – led by Dutch astronomer Peter Jonker – now thinks they have figured it out. According to them, conventional telescopes have a bias against massive black holes. In other words, our telescopes have a blind spot. And the massive black holes are right in the middle of that.

Dusty

Massive black holes can also be observed through their companion star, Jonker and colleagues say in the magazine The Astrophysical Journal. Only the circumstances work against our telescopes. The massive black holes are said to be formed from stars that implode instead of explode. As a result, the resulting massive black holes sit in the plane of the Milky Way, where they are enveloped in dust. On the other hand, lighter black holes – which our telescopes have already observed – arise from a star explosion in which they are ejected directly from the Milky Way plane and are no longer obscured by dust.

Supervisor is too far away

But the dusty environment that contains massive black holes isn’t the only reason our telescopes can’t detect these massive black holes. Jonker and colleagues argue that the precursors of heavy black holes are also very large, so that the companion star is by definition somewhat further away. When that precursor implodes, the distance to the companion remains unabated and it is much more difficult for the resulting massive black hole to snatch matter from that companion star. And that makes it even more difficult for our telescopes – which can also see massive black holes only when they ‘eat’ – to find them.

Here you can see what the different detectors and telescopes have found so far. What is striking is that dozens of heavy black holes have already been discovered via gravitational waves. It should be noted that it can be argued that gravitational wave detectors have a bias in favor of heavy black holes, because these black holes produce stronger waves when they collide than, for example, light black holes. The image also reveals that telescopes hunting for electromagnetic radiation have so far only spotted light black holes (below 20 solar masses).

The reasoning of Jonker and colleagues suggests that we have no chance and can never observe massive black holes. But that’s not true. The James Webb Space Telescope will be launched next month. This infrared telescope can see right through the dust in the Milky Way disk and should – if Jonker and colleagues’ reasoning is correct – have a significantly better chance of spotting a massive black hole.