Research by UP-led team reveals eating habits of black holes

14 April 2021 by Dr Jack Radcliffe

Research by an international team of scientists led by the University of Pretoria’s (UP) Dr Jack Radcliffe has clarified the eating habits of massive black holes.

The team, which included astronomers Professors Peter Barthel (University of Groningen, Netherlands) and Professor Michael Garrett (University of Manchester, UK), concluded that central black hole growth can occur in many kinds of galaxies. “They feed themselves in various ways: some gobble as much as they can, others digest slowly, and others are starving for food.”

This research is important for multiple reasons: It is now well established that there’s an intimate link between the creation of new stars and the growth of the central black hole within the same galaxy. For this to occur, there must be some interaction or ‘feedback’ between the two. This comes in the form of bright radiation from the central black hole. This is a crucial area of research in astronomy, as the Universe we see today is dependent upon the interaction between these two processes. This research is important as it shows that new-generation radio telescopes, such as MeerKAT (in the Karoo desert, SA), will play a huge role in disentangling the link between these two processes.

“However, we show, rather controversially, that some of these objects will still remain indetectable in such studies and show, for the first time, that the radio jets – comprising of fast-moving particles travelling near the speed of light from the black hole and a crucial component of this ‘feedback’ mechanism – are optional,” said Dr Radcliffe.

A black hole is a region of space that is so dense that even light cannot escape. While most black holes are formed when massive stars die, the ones in the centre of galaxies (as seen by the black hole image of M87 by the Event Horizon Telescope) have been growing larger and larger over many billions of years by either merging with other black holes, or eating streams of gas and dust, much of which are ripped off stars.

The research was published as a series of two papers in the international journal Astronomy & Astrophysics. It entailed a systematic study of the occurrence and nature of nuclear activity in galaxies. The team investigated issues including: ‘Which types of galaxies do the central black holes start eating?’; ‘How does this manifest itself?’; and ‘What is the best way to detect these eating phases in galaxies?’.

Caption: Dr Jack Radcliffe

Dr Radcliffe, a South African Radio Astronomy Observatory postdoctoral fellow, explained that, “Occasionally a growth phase occurs simultaneous with a star-formation phase, and is then difficult to detect. The nuclear growth process may or may not generate radio jets. Taken together, the absorption of matter onto the central black hole appears to be a standard ingredient in the life of a galaxy: black holes love food, but they eat in different ways.” He finished his PhD on this research in 2019, with professors Barthel and Garrett as his supervisors.

Occasionally an accretion or eating phase occurs simultaneously with stars being created which is then difficult to detect; the nuclear accretion process may or may not generate radio jets. These jets are important in the evolution of galaxies because they heat gas, which can then prevent stars from being formed. On the other hand, the same jets can also cause turbulence, which causes gas to collapse to form new stars. As a result, these jets can essentially regulate how many stars are formed.

“Observations with space telescopes have taught us a lot about active galaxies, but such telescopes are expensive,” Dr Radcliffe said. “We have shown that the radio telescopes of today and the coming decade, such as the Square Kilometre Array, based in SA and Australia, are optimally suited to study the eating habits of black holes in galaxies.”

According to professors Barthel and Garrett, “The evidence for massive black holes in the nuclei of all galaxies is now very strong. These black holes grow to their current mass, and it appears that our radio studies permit unique observations to address all aspects of the accretion processes, and to ultimately understand them.”

Galaxies are the building blocks of the universe. All galaxies, including the Milky Way, harbour supermassive black holes with masses millions or billions of times that of our own sun. Galaxies make stars continuously, from diffuse gas clumping together to form new stars, and occasionally during a short but rapid stellar birth wave. At the same time, supermassive black holes at the centre of galaxies grow, during relatively short phases, when they swallow matter from their immediate environment. These growth episodes manifest themselves as violent phenomena: emitting extremely strong radiation that we can then detect with our telescopes.

Astronomers have studied active galaxies since the 1950s, and gave them exotic names such as quasars, radio galaxies, and blazars. Decades of study with all kinds of telescopes on the ground and in space have yielded a good, but still not complete, picture of active galaxies.

The astronomers focused their study on a special part of the sky, the so-called GOODS-North field. That field, in size about one fifth of the area of the full moon, displays tens of thousands of faint distant galaxies, of which already quite a lot was known through studies with large optical telescopes as well as the Hubble, Spitzer and Chandra space telescopes.

Earlier, the team of astronomers used a transcontinental network of ultra-sensitive radio telescope to discover the active galaxies among all the galaxies in the field, and they published the results of that search in 2018. “Their suspicion that this technique is uniquely suited to detect nuclear activity in galaxies appeared true, but also demonstrated the existence of a class of active galaxies which are seemingly invisible to radio telescopes,” Dr Radcliffe said. 

The publications ‘Nowhere to hide: radio-faint AGN in the GOODS-N field and ‘The radio emission from Active Galactic Nuclei, by the astronomers J.F. Radcliffe, P.D. Barthel, M.A. Garrett, R.J. Beswick, A.P. Thomson, and T.W.B. Muxlow will appear this autumn in the international journal Astronomy & Astrophysics.


Scientific papers

"Nowhere to Hide: Radio-faint AGN in the GOODS-N field". By: J.F. Radcliffe et al. Accepted for publication in Astronomy & Astrophysics.


Free preprint:

"The radio emission from Active Galactic Nuclei". By: J.F. Radcliffe et al. Accepted for publication in Astronomy & Astrophysics.


Free preprint:


Artist’s impression of a galaxy with an active nucleus, a supermassive black hole, in the centre. When the black hole swallows matter, two powerful jets can form at the edges of the black hole. These jets form gigantic 'radio clouds' that can be detected by radio telescopes. (c) ESA/C. Carreau