‘Troublesome’ radio galaxy 32 times the size of Milky Way spotted

Astronomers have discovered an extraordinary new giant radio galaxy with plasma jets 32 times the size of our Milky Way.

Measuring 3.3 million light-years from end-to-end, the cosmic megastructure was spotted by South Africa’s MeerKAT telescope and nicknamed Inkathazo—meaning “trouble” in the African Xhosa and Zulu languages—because of the difficulty in understanding the physics behind it.

Researchers hope their “exciting and unexpected discovery,” published in Monthly Notices of the Royal Astronomical Society, will shed light on the mysterious origin and evolution of what are some of the largest structures in the universe.

Giant radio galaxies (GRGs) are cosmic behemoths spewing jets of hot plasma millions of light-years across intergalactic space. These plasma jets, which glow at radio frequencies, are powered by supermassive black holes at the centers of galaxies.

Until recently, GRGs were thought to be quite rare. However, a new generation of radio telescopes, such as South Africa’s MeerKAT, have since turned this idea on its head.

“The number of GRG discoveries has absolutely exploded in the past five years thanks to powerful new telescopes like MeerKAT,” said Kathleen Charlton, a Master’s student at the University of Cape Town and the first author of the new study.

“Research into GRGs is developing so rapidly that it’s becoming hard to keep up. It’s incredibly exciting.”

She added, “We nicknamed this giant galaxy ‘Inkathazo,’ meaning ‘trouble’ in isiZulu and isiXhosa because it has been a bit troublesome to understand the physics behind what’s going on here.

“It doesn’t have the same characteristics as many other giant radio galaxies. For example, the plasma jets have an unusual shape: rather than extending straight across from end-to-end, one of the jets is bent.”

Inkathazo also lives at the very heart of a cluster of galaxies, rather than in relative isolation, which should make it difficult for the plasma jets to grow to such enormous sizes.

“This is an exciting and unexpected discovery,” said Dr. Kshitij Thorat, a co-author of the study from the University of Pretoria.

“Finding a GRG in a cluster environment raises questions about the role of environmental interactions in the formation and evolution of these giant galaxies.”

To try and understand more about this cosmic conundrum, the researchers took advantage of MeerKAT’s exceptional capabilities to create some of the highest-resolution spectral age maps ever made for GRGs. These maps track the age of the plasma across different parts of the GRG, providing clues about the physical processes at work.

The results revealed intriguing complexities in Inkathazo’s jets, with some electrons receiving unexpected boosts of energy. The researchers believe this may occur when the jets collide with hot gas in the voids between galaxies in a cluster.

“This discovery has given us a unique opportunity to study GRG physics in extraordinary detail,” said Thorat. “The findings challenge existing models and suggest that we don’t yet understand much of the complicated plasma physics at play in these extreme galaxies.”

Most known GRGs have been found at northern latitudes with European telescopes, while the southern sky remains relatively unexplored for such giant objects. Yet Inkathazo is not alone. It is the third GRG to be spotted in a small patch of sky, around the size of five full moons, that astronomers refer to as “COSMOS.”

When an international team of astronomers named the “MIGHTEE” collaboration observed COSMOS with the MeerKAT telescope, they immediately spotted the other two other GRGs and published their findings in 2021.

Inkathazo was seen more recently in follow-up observations with MeerKAT, which is operated by the South African Radio Astronomy Observatory.

“The fact that we unveiled three GRGs by pointing MeerKAT at a single patch of sky goes to show that there is likely a huge treasure trove of undiscovered GRGs in the southern sky,” said Dr. Jacinta Delhaize, a researcher at the University of Cape Town, who led the 2021 publication.

“MeerKAT is incredibly powerful and in a perfect location, so is excellently poised to uncover and learn more about them.”

As a precursor to the Square Kilometer Array (SKA) due to begin operations at the end of this decade, MeerKAT offers unprecedented sensitivity and resolution, enabling discoveries like Inkathazo.

“We’re entering an exciting era of radio astronomy,” said Dr. Delhaize. “While MeerKAT has taken us further than ever before, the SKA will allow us to push these boundaries even further and hopefully solve some of the mysteries surrounding enigmatic objects like giant radio galaxies.”