6 mins read 14 Feb 2022

New Method To Detect Active Black Holes in the Universe

Black holes of any kind are incredibly hard to find, even with the most powerful telescopes. Australian astronomers have now developed a new technique that can identify active black holes in millions of galaxies.

At its heart, though not visible here, is a supermassive black hole behaving in a strange and unexpected manner. Researchers have detected a clumpy gas stream flowing quickly outwards and blocking 90% of the X-rays emitted by the black hole. This activity could provide insights into how supermassive black holes interact with their host galaxies. Credit: ESA/Hubble, A. Riess et al., J. Greene.

Astronomers from the International Centre for Radio Astronomy Research (ICRAR) in Western Australia recently found a way to find more supermassive black holes in the Universe, utilising a new technique that models emissions from galaxies and active black holes at different wavelengths of light. 

The new technique, which can now be applied to millions of galaxies, can reveal a little more about how these supermassive black holes are interacting with their local environments, and what effect these behemoth objects have on the lives and evolution of their host galaxies.  

Lead author and PhD student from the University of Western Australia (UWA), Jessica Thorne, developed a new algorithmic technique to allow astronomers to identify these events from existing data.  “We can identify these active black holes and look at how much light they’re emitting, but also measure the properties of the galaxy it is in at the same time,” Thorne said.

Astronomers call this phenomenon, when the centre or nucleus of a galaxy becomes more luminous than the galaxy itself, active galactic nuclei (AGN) and they are one of the most energetic types of object that we know of; some AGN radiate more than a thousand billion times the brightness of our Sun!

“The black holes we’re looking for are between a million and a billion times more massive than our Sun,” said Thorne.

“As they suck in matter from around them, the matter gets super-heated because of friction and becomes very, very luminous.”

“And when they’re active, these black holes can outshine the rest of the galaxy,” she said.

The work, published in the Monthly Notices of the Royal Astronomical Society, describes how the researchers were able to take optical, ultraviolet and infrared data from roughly 750 million galaxies and successfully identify over 77,000 AGN. This new method, which models the distribution of different wavelengths of light coming from around black holes and their host galaxies, was found to have a 91% success rate at correctly recognising known AGN.

Another scientist in the same ICRAR-UWA group, Dr Sabine Bellstedt, said that this new technique will enable astronomers to better study black holes and the evolution of galaxies and that “it suddenly means we can look for active black holes in so many more places than we were able to before“.  

Identifying Active Galactic Nuclei (AGN)

This AGN unification diagram shows the geometry and characteristics of different types of AGN. For each subclass of AGN we can predict whether they are likely to be radio-loud or quiet and whether they'll have low or high electromagnetic power at different wavelengths. Credit: J.E. Thorne.

If you’ve ever had the enchanting experience of catching the hazy swirls of a distant galaxy being observed by a telescope, you might have found yourself wondering what holds galaxies together. The short answer is gravity but unlike the gravitational pull of a planet or star, like Earth’s effect on the Moon or the Sun’s hold on the planets, the source of the force that keeps billions of stars in a galactic orbit is one that we can’t usually see - a supermassive black hole. Technically, and as Einstein showed us, it’s actually the curvature of space-time that creates gravity, and the larger the mass, the greater that curvature will be.

Generally, black holes (of all types) aren’t normally seen because their immense gravity stops everything - even light - from escaping its grasp if it strays too close. Naturally, this makes these spectacularly dense objects somewhat difficult to study, but something that astronomers can observe is the enormous mess black holes make when they eat their stellar lunches or the violent signature they leave on their surrounding regions.

Nearly all spiral galaxies (like our Milky Way) have supermassive black holes (SMBHs) - these are black holes with between a million and a billion times the mass of our Sun - at their centre. When gas and other stellar matter is pulled towards a SMBH it forms a disk of material (known as an accretion disc) that gets super-heated by friction and gravity, resulting in exceptionally bright electromagnetic radiation from the region.  

A mosaic of some of the galaxies involved in the study. Credit: GAMA Survey Team-ICRAR/UWA, and the KiDS and VIKING Teams.

The researchers developed this new technique by using an algorithm called ProSpect to model emission from galaxies and black holes at different wavelengths of light.

They then applied the method to almost half a million galaxies from Anglo-Australian Telescope’s DEVILS survey. They also applied it to more than 200,000 galaxies from the GAMA survey, which brings together observations from six of the world’s best ground and space-based telescopes.  

Dr Sabine Bellstedt said scientists often fail to account for bright black holes in galaxies.

“One of the reasons we’ve ignored them in the past is because it’s hard to find them all,” she said. “We don't really understand these bright black holes to incorporate them into our modelling with sufficient detail.”

“It suddenly means we can look for active black holes in so many more places than we were able to before - it’s going to help us search more galaxies, and look further back in time to the distant Universe.”

Naturally, the existence of an active SMBH has an enormous impact on the way galaxies change over time as their insatiable appetites prevent the growth of the galaxy and slow the formation of new stars. Thorne even suggested that hosting an AGN in a galaxy “can effectively kill it”, which would support the claim that studying these luminous objects is critical to our understanding of the ways all galaxies live and die.

“We think that an active black hole in a galaxy is able to decrease the amount of star formation really quickly and stop the galaxy from growing any further,” Thorne said.

“It can effectively kill it.”

With observations from new telescopes such as the James Webb Space Telescope, the Vera C. Rubin Observatory in Chile, and the Square Kilometre Array in Australia and South Africa, astronomers may be able to apply the technique to millions of galaxies at once.

“It’s exciting to think about how many doors this has unlocked for the future,” Thorne said.