Pune: Astronomers from Pune have identified a giant radio galaxy that has been through four separate episodes of activity from its central supermassive black hole — the first known galaxy of its kind — offering a rare window into how black holes switch on and off over millions of years.
The galaxy, catalogued as J023721.13-010528.5, stretches 4.2 million light-years across and has a distinctive S-shaped structure. Its discovery was reported by PhD student Pavan Vijay Khadekar and his supervisors, Professor Dharam Vir Lal of the National Centre for Radio Astrophysics (NCRA-TIFR) and Professor Ramana Athreya of the Indian Institute of Science Education and Research (IISER) Pune, in The Astrophysical Journal.
The paper was published on October 21 by IOP Publishing on behalf of the American Astronomical Society, and carries the digital object identifier 10.3847/1538-4357/aea20c.
Radio galaxies are galaxies whose central supermassive black holes power enormous jets of material that stream outwards into space at close to the speed of light. These jets glow brightly at radio wavelengths and can extend for millions of light-years, dwarfing the galaxy itself. In many cases, the jets eventually fade and the galaxy's radio activity appears to stop.
In some galaxies, however, the black hole becomes active again, producing a fresh set of jets inside the fading remnants of the old ones. Such galaxies are known as "restarted" radio galaxies, and they are prized by astronomers because they preserve a record of a black hole's past lives, layered one inside another like the growth rings of a tree.
Until now, only six radio galaxies were known to show evidence of three distinct periods of activity. The Pune team's galaxy is the first to show four. That distinction matters because each additional episode adds a chapter to the story of how black holes feed, fall silent and reignite — and because finding even one four-episode galaxy suggests that more may be waiting to be found, possibly with even more cycles.
Khadekar came across the object while studying radio galaxies with unusual shapes. Its S-shaped structure caught his attention, and the evidence for a fourth period of activity emerged unexpectedly as the team analysed observations from several radio telescopes. Images assembled from observations at multiple frequencies revealed the traces of different jet episodes layered over one another, like overlapping ripples frozen in time.
The researchers used high-quality observations taken by the Giant Metrewave Radio Telescope (GMRT) in India, along with archival data from the Very Large Array in the United States and MeerKAT in South Africa. By combining data from three continents, the team was able to separate the signatures of each episode and date them.
They concluded that the galaxy's current, fourth episode of activity began a few million years ago, while its first episode ended around 21 million years ago. The jet cycles, in other words, span roughly 6 to 21 million years — timescales that are vast by human standards but brief in the life of a galaxy.
Each episode leaves behind a set of bright hotspots where the jets slam into the surrounding intergalactic gas. The team spotted four sets of such hotspots, the tell-tale fingerprints of four separate awakenings. The oldest hotspots are the faintest and most spread out, having had millions of years to fade and expand.
The newest are bright and compact, close to the galaxy's centre, marking the black hole's latest outburst. Reading those hotspots in sequence allows astronomers to reconstruct a history that no single observation could capture.
Khadekar said that data made freely available by radio observatories allow researchers around the world to examine the same observations from different perspectives. That collaborative approach, he said, supports more comprehensive studies and makes wider use of telescope data — a point that underlines how modern astronomy increasingly depends on open archives as much as on new observations.
The GMRT data, the American archival data and the South African data each contributed a piece of the puzzle, and none alone would have revealed all four episodes.
Professor Lal said galaxies with multiple periods of activity help researchers understand the processes behind radio emission and how those processes change over time. The newly identified galaxy, he noted, is the first known to show four episodes, adding to the small group of restarted radio galaxies. The study also proposes a strategy that could help astronomers find more such objects, including galaxies with an even greater number of activity cycles.
If the method works, the census of restarted galaxies could grow rapidly, and with it our understanding of the duty cycle of black holes — the fraction of their lives they spend actively feeding and firing jets.
Professor Divya Oberoi of NCRA-TIFR, who was not involved in the study, said radio galaxies are valuable both as objects of study and as tools for investigating fundamental questions about the origin of the universe.
She noted that the GMRT, a low-frequency radio telescope built and operated by NCRA, a unit of the Tata Institute of Fundamental Research, was developed to support Indian astronomers' work at the highest level and to contribute to the global effort to understand the cosmos. The new discovery, she suggested, is an example of that ambition being realised.
The research received support from the Government of India through grants to IISER Pune and NCRA-TIFR. It is also a reminder of Pune's quiet but significant place in world astronomy. The GMRT, located at Khodad in Pune district, is one of the world's most sensitive low-frequency radio telescopes, and it has played a role in discoveries ranging from distant galaxies to pulsars and the cosmic dawn. The new galaxy adds another entry to that list.
Why does a galaxy with four lives matter beyond astronomy? Because supermassive black holes are not passive curiosities. Their jets inject enormous amounts of energy into their surroundings, heating gas, suppressing star formation and shaping the evolution of entire galaxies and clusters.
Understanding when and why they switch on and off is therefore central to understanding why galaxies look the way they do today. A galaxy that has switched on four times is a natural laboratory for that question, preserving four experiments in a single object.
The discovery also raises new questions. What triggers a dormant black hole to reignite? Is it a fresh infall of gas, a merger with another galaxy, or an instability in the accretion disc swirling around the black hole?
How long does each episode last, and how long are the silences in between? And are four-episode galaxies genuinely rare, or have astronomers simply not yet learned to recognise them? The Pune team's proposed search strategy is aimed at answering that last question first — by looking systematically for the layered signatures that betray multiple lives.
For now, J023721.13-010528.5 stands alone as the first galaxy known to have lived four radio lives. It is a discovery made by a PhD student, two supervisors and three telescopes on three continents, and it shows how much history can be hidden in a single smudge of radio light — if you know how to read it.
(Reporting based on Pune Pulse and NewsBytes.)