When the most massive stars die, they collapse to form some of the densest objects known in the universe: black holes. They are the “darkest” objects in the cosmos, as not even light can escape their incredibly strong gravity.

Because of this, it is impossible to directly image black holes, making them mysterious and quite perplexing. But our new research has road-tested a way to spot some of the most voracious black holes of all, making it easier to find them buried deep in the hearts of distant galaxies.

Despite the name, not all black holes are black. While black holes come in many different sizes, the biggest ones are at the centres of galaxies and are still growing in size.

These “supermassive” black holes can have a mass of up to a billion Suns. The black hole at the centre of our own Milky Way galaxy – called Sagittarius A*, whose discovery received the 2020 Nobel Prize in Physics – is fairly calm. But that is not the case for all supermassive black holes.

If materials such as gas, dust or stars get too close to a black hole, it gets sucked in by the enormous gravitational force. As it falls towards the black hole, it heats up and becomes incredibly bright.

The light produced by these “bright black holes” can span the entire electromagnetic spectrum, from X-rays to radio waves. Another name for the bright black holes at the centre of galaxies is “active galactic nuclei”, or AGN. They can shine trillions of times brighter than the Sun, and can sometimes even outshine all the stars in its galaxy.

Matter swirling into the supermassive black hole at the centre of M87. Photo credit: Event Horizon Telescope

Brightest black holes

Some active galactic nuclei violently spew out matter via a jet, which travels millions of kilometres through space and can be seen by radio telescopes. Others produce “winds” at the centre of the galaxy, capable of pushing any gas (the fuel needed for stars to form) out of the galaxy.

Violent jets spewing from Hercules A. Photo credit: NASA/ESA/NRAO

With such destructive forces in the middle of a galaxy, astronomers are certain this must have a big impact on the galaxy itself. We know most galaxies are slowly turning off their star formation processes and active galactic nuclei might be one of the culprits.

Active galactic nuclei can therefore not only help us to better understand elusive black holes but studying them also teaches us about galaxies themselves.

Finding black holes

Depending on how much a black hole is “eating”, what galaxy it is in and the angle from which we can see it, active galactic nuclei can look very different to one another. Even when looking at the same galaxy, one astronomer with an X-ray telescope may see it glow and discover an active galactic nucleus, whereas another astronomer using a radio telescope might see nothing, if the active galactic nuclei do not happen to produce jets that are visible in the radio spectrum.

Because of this, it was thought they were all different objects, but by looking at the same objects with different telescopes astronomers discovered they had many similarities and realised the benefits of using more of the electromagnetic spectrum to find them.

The relative brightness of a galaxy across different parts of the electromagnetic spectrum is called its “spectral energy distribution”. This can be used to measure how many stars are in a galaxy, how old they are, what they are made of and how much dust is blocking the light.

Composite picture showing how a typical galaxy appears at different wavelengths. Photo credit: ICRAR/GAMA and ESO

In our research, published today in Monthly Notices of the Royal Astronomical Society, we show that this technique can also be used to spot active galactic nuclei. This means we can now measure not just the properties and histories of the stars in the galaxy, but also the brightness of its central black hole.

It is not a simple thing to do. The difference between starlight and the light from an active galactic nucleus is incredibly subtle, so it is possible to confuse young stars for a bright black hole and vice versa.

Here in Australia, astronomers have been using Australian telescopes to make 3D maps of galaxies in specific patches of the sky. These maps let us scour hundreds of thousands of galaxies, spanning 11 billion years of history, for possible active galactic nuclei.

By applying our new method to 700,000 galaxies we identified and quantified more than 75,000 active galactic nuclei to begin understanding how their number has evolved over time and how they have impacted their host galaxies. Astronomers think the number of active galactic nuclei in the universe is linked to the amount of star formation, which we know was almost ten times higher roughly 10 billion years ago. But until we can be certain we have identified all the active galactic nuclei across cosmic time in our galaxy samples, we will not know for sure.

Right now, the astronomical community is still passionately debating the nature of active black holes. While we have not yet answered the questions needed to soothe the debate, we are now one step closer to reliably being able to spot these fascinating objects within galaxies. And that is an important step towards shedding more light on the mystery of black holes.

Jessica Thorne is an Astrophysics PhD Candidate and Sabine Bellstedt is a Research Associate in Astronomy at The University of Western Australia.

This article first appeared on The Conversation.