An international team of astronomers, led by University of Pretoria (UP) researchers, has provided deeper insight into the life cycle of giant radio galaxies.

A breakthrough study, enabled by supercomputing, is challenging known theoretical models by explaining how extragalactic cosmic fountains grow to cover colossal distances. The study has also raised new questions about the mechanisms behind these vast cosmic structures.

The study’s findings were published in the journal Astronomy & Astrophysics.

Giant radio galaxies resemble ‘cosmic fountains’ – jets of superheated gas that are ejected into near-empty space from their spinning supermassive black holes.

The research team – led by astrophysicist Dr Gourab Giri, who holds a postdoctoral fellowship from the South African Radio Astronomy Observatory at UP – tackles a key question in modern astrophysics: How these structures, which are larger than galaxies and are made up of black hole jets, interact over cosmological timescales with their thin, gaseous surroundings.

“We mimicked the flow of the jets of the fountains in the universe to observe how they propagate themselves over hundreds of millions of years – a process that is, of course, impossible to track directly in the real cosmos,” Giri explains.

“These sophisticated simulations enable a clearer understanding of the likely life cycle of radio galaxies by revealing the differences between their smaller, early stages, and giant, mature stages. Understanding the evolution of radio galaxies is vital for deepening our knowledge of the formation and development of the universe.”

A UP associate professor, Kshitij Thorat, adds: “While such studies are computationally expensive, the team embarked on this adventure informed by the exciting, cutting-edge observations carried out by new-generation radio telescopes, such as the South African MeerKAT telescope, which has been instrumental in providing us with the details of the structure of these cosmic fountains.”

Supermassive black holes ‘wake up’

Astronomers study galaxies for more than just the stars they can see, Giri says.

“We also look at many, often interrelated, phenomena. One of the most amazing things to see is when a supermassive black hole at the centre of a galaxy, which is relatively tiny in size compared to the galaxies they grow in, ‘wakes up’ and starts eating up lots of nearby gas and dust.

“This isn’t a calm, slow or passive process. As the black hole pulls in material, the material gets superheated and is ejected from the galaxy at near-light speeds, creating powerful jets that look like cosmic fountains. These fountains emit radio signals as the accelerated high-speed plasma matter generates radio waves. These signals are detected by very powerful radio telescopes, built through the collaborative efforts of multiple countries working together.”

With the recent advent of powerful and sensitive radio telescopes – such as MeerKAT, the Low-frequency Array in Europe and the Giant Metrewave Radio Telescope in India – astronomers are now detecting these fountains in their faintest stages, says Giri.

“These advanced telescopes can capture the weakest signals from dying or fading parts of the jet, leading to discoveries of more such extended sources that were previously undetectable.” 

Cosmic jets can travel 16 million light-years

The study also implies that giant jets may be more common than previously thought.

Since the discovery of these high-speed fountains in the 1970s, astronomers have been curious about how far the ejected matter travels before eventually fading out. The answer was astounding as they began to discover that cosmic jets travel vast distances – some reaching nearly 16 million light-years (nearly six times the distance between the Milky Way and Andromeda).

“I took on the challenge of developing theoretical models for these sources, rigorously testing the models with the advanced capabilities of modern supercomputers,” says Giri.

“This computer-driven study aimed to simulate the behaviour of giant cosmic jets within a mock universe, constructed according to known physical laws governing the cosmos. Our primary focus was to answer two questions: Is the enormous size of these jets due to their exceptionally high speeds, or is it because they travel through regions of space that are nearly empty of surrounding matter, offering minimal resistance to the jets’ free propagation?”

The study presents evidence that a combination of these considerations is a key aspect in the formation of these giant jets.

With the help of the supercomputing power of the Inter-University Institute for Data Astronomy – a collaborative network consisting of UP, the University of Cape Town (UCT) and the University of Western Cape – the international research team was able to analyse the vast quantities of simulated data, effectively spanning millions of years.

“These computer-based models, which simulate jet evolution in a mock universe, do more than explain the origin of most giant radio galaxies,” says Giri

“They’re also powerful enough to address puzzling exceptions that have confused astronomers in this field. For example, they help explain how some cosmic fountains bend sharply, forming the shape of an X in radio waves instead of following a straight path, and clarify the conditions under which giant fountains can still grow in dense cosmic environments.”

These findings can be tested further by radio astronomers using advanced telescopes.

“Studies like this lead the way in formulating our understanding of these wonderful objects from a theoretical perspective,” says Thorat.

The research team consisted of Thorat and Extraordinary Prof Roger Deane from UP’s Faculty of Natural and Agricultural Sciences; Prof Joydeep Bagchi from Christ University in India; Prof DJ Sailkia from the Inter-University Centre for Astronomy and Astrophysics, also in India; and Dr Jacinta Delhaize from UCT.

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