GMRT (Giant Metrewave Radio Telescope) discovers the oldest known fossil radio galaxy trapped inside a cluster of galaxies
GMRT Reveals 260 Million Year Old Fossil Radio Lobes in Abell 980 Galaxy Cluster
GMRT (Giant Metrewave Radio Telescope) discovers the oldest known fossil radio galaxy trapped inside a cluster of galaxies — An extremely old remnant of the ‘lobes’ of a once active radio galaxy has been discovered by an Indian team of astronomers led by Dr. Surajit Paul of Savitribai Phule Pune University (SPPU) with the help of GMRT. Because of the high sensitivity of the GMRT, the discovery of these "fossil lobes" via their low radio-frequency radiation was made possible.
A remarkable radio astronomy discovery made with India’s Giant Metrewave Radio Telescope (GMRT) has revealed extremely old fossil radio lobes inside the galaxy cluster Abell 980. The diffuse radio structures are estimated to be about 260 million years old and may preserve evidence of an earlier episode of activity from the supermassive black hole at the centre of the cluster’s brightest galaxy.
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What makes the system particularly interesting is that the old radio lobes are not simply aligned with the younger radio lobes seen near their apparent parent galaxy. Researchers have proposed that the galaxy may have moved toward the centre of the cluster between two major episodes of jet activity, producing an unusual structure known as a Detached-Double-Double Radio Galaxy (dDDRG).
The discovery and its interpretation were reported in two scientific papers published in 2022 in prestigious international journals - Astronomy & Astrophysics (A&A) and in the Publications of the Astronomical Society of Australia (PASA).
In addition to Dr. Surajit Paul, the other researchers included Sameer Salunkhe, Dr. Satish Sonkamble, Shubham Bhagat, and Prof. Gopal Krishna.
The team said that this pair of gigantic lobes of a radio galaxy spanned 1.2 million light-years inside the galaxy cluster Abell 980 and was created about 260 million years ago.
Important: The estimated age of about 260 million years refers to the old radio-emitting structures and their associated jet activity—not to the age of the entire galaxy.
What Did GMRT Discover?
Astronomers studying the galaxy cluster Abell 980 (A980) identified two large, diffuse radio sources with extremely steep radio spectra.
These sources, labelled A and B in the research, extend toward opposite sides of the cluster's hot intracluster medium. Their properties are consistent with the interpretation that they are relic or fossil radio lobes left behind by an earlier episode of activity from the brightest cluster galaxy (BCG). Their estimated spectral age is about 260 million years.
The older radio-lobe pair spans roughly 350 kiloparsecs, equivalent to about 1.14 million light-years, consistent with the approximately 1.2-million-light-year scale reported in contemporary coverage of the discovery.
The discovery is significant because such old radio structures normally become extremely faint and difficult to detect.
Note:
1 parsec (pc) ≈ 3.26 light-years
1 kiloparsec (kpc) = 1,000 parsecs
So, 1 kpc ≈ 3,260 light-years
So, 350 kpc ≈ 1.14 million light-years
Is This a 260-Million-Year-Old Galaxy?
No.
The phrase “260-million-year-old fossil radio galaxy” can easily be misunderstood as meaning that the entire galaxy is 260 million years old.
That is not what the research established.
The approximately 260-million-year age is an estimated spectral age of the old ultra-steep-spectrum radio sources, which are interpreted as relic radio lobes produced during an earlier episode of jet activity.
The parent galaxy itself is much older. The study is essentially looking at the radio fossil record of an earlier phase of activity associated with its central supermassive black hole.
A more scientifically precise description is therefore:
“Approximately 260-million-year-old fossil radio lobes in Abell 980.”
What Are Fossil Radio Lobes?
To understand the discovery, it helps to first understand what a radio galaxy does.
Large galaxies are made up of billions of stars. A lot of them reside in clusters containing 100 to 1000 galaxies that are all connected together by mutual gravity.
Additionally, all large and massive galaxies are currently thought to harbor a supermassive black hole (SMBH) at their center, often with a mass equivalent to several million to billions of Suns.
When the black hole's surrounding environment becomes active, powerful relativistic jets can be launched in opposite directions.
These black holes emit two oppositely-directed collimated "jets" of magnetised relativistic plasma when they become "active," each of which feeds into an expanding lobe that makes it radiate at radio frequencies.
These jets carry energetic particles and magnetic fields far beyond the central galaxy. As the jets interact with the surrounding environment, they can inflate enormous regions of radio-emitting plasma known as radio lobes.
Large radio telescopes can detect such "radio lobes" up to a distance of billions of light-years.
A supermassive black hole's episode of jet production in a galaxy (called the 'active' phase) typically lasts tens of millions of years, after which jet production and hence energy supply into the radio lobes ceases. Then, the two radio lobes rapidly fade away without being detected. In fact, it's possible that billions of massive galaxies blew out radio lobes during their multiple active phases over the age of the universe and the universe is infested with their faded relics.
[Caption: Optical photograph of the cluster of galaxies, whose central giant elliptical galaxy (white elliptical patch) is the parent of the radio lobes shown in red. The blue-shaded region displays the halo of X-ray emission due to the hot gas associated with this cluster of galaxies which have an overall size of around 1.7 million light-years.]
The basic process is:
Supermassive black hole
↓
Active phase
↓
Relativistic jets
↓
Large radio lobes
↓
Jet activity declines or switches off
↓
Radio lobes gradually fade
↓
Fossil/relic radio lobes
Even after the central engine becomes less active, the old lobes can remain detectable for some time.
In special environments, such as galaxy clusters filled with hot plasma, their detectable lifetime can be considerably extended.
Why Are Fossil Radio Lobes Difficult to Find?
Radio lobes do not remain bright forever — Relics or fossil lobes of radio galaxies are also repositories of important information about the circumstances in the bygone era of the universe, just like the fossil records of animals, plants, and terrestrial events. Sadly, the rapid fading of the fossil radio lobes makes it difficult to discover them.
When the jets stop supplying fresh energy, the relativistic electrons inside the lobes lose energy and the radio emission becomes progressively weaker.
The radio spectrum also becomes increasingly steep as the population of energetic electrons ages.
This makes old radio lobes particularly challenging to detect.
The situation changes when the parent galaxy lies inside a galaxy cluster.
Galaxy clusters contain extremely hot, diffuse gas known as the intracluster medium (ICM). The pressure of this surrounding gas can help confine radio plasma and slow the expansion of the old lobes.
That can allow ancient radio structures to remain detectable for much longer than they otherwise might.
(In other words, the pressure of the surrounding hot gas, which typically permeates galaxy clusters and is detectable with X-ray telescopes, can significantly slow down the expansion and subsequent fading of the radio lobes if the parent galaxy happens to be inside a cluster of galaxies. This external confinement of the radio lobes during their relic stage can greatly extend their detectable lifespan, particularly at meter wavelengths where the radiative losses are quite low.)
The galaxy cluster harboring the fossil radio lobes must remain in a dynamically quiescent condition, in order for the lobes to be detectable despite their lengthy lifespan. This is the second crucial criterion for their detection. Luckily, the cluster Abell 980 is in a relaxed state as indicated by its thermal X-ray emission.
The A980 study found that the cluster has a relatively relaxed, cool-core environment surrounded by an extensive hot ICM, making it an unusually favourable setting for preserving old radio structures.
Why Was Abell 980 a Good Place to Find Ancient Radio Fossils?
Abell 980 is a galaxy cluster containing a large amount of hot intracluster gas.
The X-ray analysis showed a cool core with a temperature of about 4.2 keV, surrounded by a hotter and more extensive ICM with a temperature of about 6.8 keV.
The cluster also shows evidence of X-ray surface-brightness discontinuities, or cold fronts, associated with the locations of the two aged ultra-steep-spectrum radio sources.
Researchers proposed that the old radio lobes rose buoyantly through the cluster's hot gas toward the outskirts of the X-ray halo.
This combination of:
- old radio plasma,
- hot surrounding gas,
- confinement by the intracluster medium,
- and a relatively undisturbed cluster environment
may have helped preserve the fossil radio lobes long enough for astronomers to detect them.
According to Dr. Paul and Prof. Gopal Krishna, the GMRT discovery of the two extremely old fossil radio lobes with a record age of nearly 260 million years (a record so far), owes to a combination of these two extraordinarily rare favorable circumstances.
Dr. Paul initiated and has long studied the study of radio-wave emission from the environments within relatively low-mass clusters of galaxies. He believes that low-mass clusters of galaxies, like Abell 980, are uniquely suited for satisfying the requirement of a less disturbed internal environment because their shallow gravitational potential is dominated by at most a few large galaxies.
How Did GMRT Detect the Ancient Radio Structures?
The discovery relied heavily on low-frequency radio observations.
The first study analysed archival observations from:
- GMRT at 150 MHz and 325 MHz
- EVLA/VLA at 1.5 GHz
- Chandra X-ray Observatory
The two large radio sources showed an ultra-steep radio spectrum, a characteristic associated with aged populations of relativistic electrons.
A later study incorporated observations from the LOFAR Two-metre Sky Survey Data Release 2 (LoTSS-2) at 144 MHz, together with existing GMRT and VLA observations.
The additional low-frequency information helped researchers investigate the morphology and spectrum of the old and young radio components in A980.
Why is GMRT Important for this discovery?
The Giant Metrewave Radio Telescope (GMRT) is one of India's most important radio astronomy facilities.
This front-ranking radio telescope is located near Khodad, about 80 km north of Pune, Maharashtra, and is operated by the National Centre for Radio Astrophysics (NCRA) of the Tata Institute of Fundamental Research (TIFR).
GMRT consists of 30 fully steerable antennas, each 45 metres in diameter, spread across a region extending up to about 25 km.
Its metre-wavelength capabilities make it particularly valuable for studying faint, extended and steep-spectrum radio emission.
The A980 research made use of archival GMRT observations at 150 and 325 MHz, which were crucial for identifying and characterising the aged radio structures.
What Makes the Radio Lobes “Ultra-Steep Spectrum”?
A radio source's spectrum describes how its radio brightness changes with observing frequency.
The A980 relic sources have an ultra-steep spectrum, with the study reporting a spectral index of approximately ╬▒ < −2.
This steep spectrum is consistent with an aged population of relativistic electrons that has undergone significant energy losses.
In simple terms:
Younger energetic radio plasma → relatively stronger high-frequency emission
Older radio plasma → progressively weaker high-frequency emission
Therefore, very steep radio spectra can provide an important clue that a radio structure is old.
The Mystery of the Missing Parent Galaxy
The discovery became even more intriguing when researchers compared the old radio lobes with the radio source associated with the cluster's brightest cluster galaxy (BCG).
The central BCG contains a much smaller and brighter double radio source, about 55 kiloparsecs in size.
This raised an important question:
Could the old outer radio lobes and the younger central radio source have been produced by the same galaxy at different times?
Researchers proposed that this may indeed be the case.
According to their model, the galaxy that produced the old radio lobes may have moved toward the centre of the cluster during the long lifetime of the relic lobes.
When the galaxy later entered another active phase, its central black hole produced a new pair of radio lobes.
The old lobes therefore remained behind while the newer pair appeared closer to the galaxy's present location.
What is a Double-Double Radio Galaxy?
A Double-Double Radio Galaxy (DDRG) is a radio galaxy showing two pairs of radio lobes associated with separate episodes of activity from the same central engine.
The concept can be visualised as:
First jet episode
→ old outer radio lobes
Central engine becomes quieter
→ old lobes age
Second jet episode
→ new inner radio lobes
This tells astronomers that the activity of a galaxy's central black hole can be episodic rather than continuous.
In other words, the black hole's powerful jet-producing phase can switch off and later become active again.
Note: Prof. Gopal Krishna and colleagues solved the mystery of the ‘missing’ parent galaxy of the two fossil radio lobes stated above by showing that their parent galaxy has drifted away towards the gravitational center of the galaxy cluster during the long lifespan of those fossil radio lobes, thus traversing a distance of 250000 light-years. That galaxy has entered into a new phase of activity on reaching the cluster core, creating a new pair of radio lobes that are both smaller and brighter. Thus, they are the younger siblings of the two fossil radio lobes.
The novel feature of this recently proposed model invoking the motion of the parent galaxy is that the younger pair of radio lobes would not be collinear with the two fossil radio lobes but rather have a huge lateral separation from them. Because of this large positional offset, the pair of younger radio lobes could be easily misconstrued as an independent double radio source unrelated to the fossil lobes.
What is a Detached-Double-Double Radio Galaxy (dDDRG)?
This is where the Abell 980 system becomes especially unusual.
In a conventional double-double radio galaxy, the older and younger radio-lobe pairs generally share a common axis.
In A980, the researchers proposed something different.
The old radio lobes and the younger radio double appear to be laterally displaced from one another, although their axes are approximately parallel.
The PASA study argues that this unusual geometry can be explained if the parent galaxy moved laterally toward the cluster centre between its major episodes of jet activity.
The researchers therefore proposed that A980 may represent a particularly plausible example of a:
Detached-Double-Double Radio Galaxy (dDDRG)
The important word here is “proposed.”
The dDDRG classification is a scientific interpretation based on the observed radio morphology, spectral properties and cluster environment. It should not be presented as an independently proven fact that the galaxy physically travelled through space in a directly observed trajectory.
How Far Could the Parent Galaxy Have Moved?
The PASA study proposes that the parent galaxy may have undergone a substantial lateral displacement between the two episodes of jet activity.
The paper discusses a displacement of roughly 70 kiloparsecs between the old and young radio structures, while the broader proposed scenario involves the parent galaxy drifting toward the cluster centre over a much larger scale.
This is an important distinction from the simplified claim that “the galaxy was observed moving 250,000 light-years.”
Astronomers are reconstructing the likely history from the present-day radio morphology, spectral properties and cluster environment.
It is therefore better to describe the movement as a model-based interpretation, rather than as a directly measured motion.
What Other Telescopes Contributed?
GMRT was not working alone.
The scientific picture of Abell 980 was assembled using observations across different parts of the electromagnetic spectrum.
GMRT
Low-frequency radio observations at 150 and 325 MHz helped reveal the diffuse, steep-spectrum radio structures.
VLA / EVLA
Higher-frequency radio observations around 1.5 GHz helped characterise the radio morphology and spectral behaviour.
LOFAR
The LoTSS-2 144-MHz observations provided additional low-frequency information and helped strengthen the proposed connection between the old and young radio sources.
Chandra X-ray Observatory
X-ray observations revealed the hot intracluster medium and helped scientists investigate the relationship between the radio structures and the surrounding cluster gas.
This is a good example of why modern astronomy often depends on multi-wavelength observations rather than a single telescope.
What Does This Discovery Tell Us About Supermassive Black Holes?
One of the most interesting implications is that powerful black-hole jet activity can be recurrent.
A supermassive black hole does not necessarily remain in one permanent state.
Its central engine can experience different episodes of activity:
Active phase
→ powerful jets
Quiet or weaker phase
→ old lobes fade
New active phase
→ another pair of jets
The Abell 980 system provides an unusual opportunity to study such recurrent activity because evidence of an older episode appears to have survived in the form of diffuse radio lobes.
Why are Galaxy Clusters Important for Radio Astronomy?
Galaxy clusters are not simply collections of galaxies. They contain a vast reservoir of extremely hot plasma. This intracluster medium can interact with radio jets and lobes, influencing:
- the expansion of radio lobes,
- their morphology,
- their buoyant motion,
- their survival,
- and their detectability.
The A980 study therefore demonstrates how the environment surrounding a galaxy can preserve information about its past black-hole activity. In this sense, fossil radio lobes act like a cosmic archaeological record.
What is the Difference Between a Radio Galaxy and a Fossil Radio Galaxy?
| Feature | Active Radio Galaxy | Fossil/Relic Radio Structure |
|---|---|---|
| Central engine | Active | May be inactive or in a later activity phase |
| Jets | Currently powered | Earlier jets have switched off or weakened |
| Radio lobes | Bright and powered | Diffuse and fading |
| Spectrum | Often less steep | Can become very steep |
| Scientific value | Shows current activity | Preserves evidence of past activity |
The A980 system is particularly interesting because it may show both older relic lobes and a younger active radio double associated with the same BCG.
Why is this Discovery Important?
The Abell 980 radio system is valuable for several reasons.
1. It preserves evidence of ancient black-hole activity
The old radio lobes may be relics of an earlier jet episode from the central galaxy.
2. It shows that radio lobes can survive for hundreds of millions of years
The hot intracluster medium may help prolong their detectability.
3. It provides evidence for recurrent jet activity
The younger radio source near the BCG may represent a later active phase.
4. It offers a possible example of a dDDRG
The unusual spatial separation between the old and young radio-lobe pairs makes A980 an especially interesting candidate.
5. It demonstrates the importance of low-frequency radio astronomy
Low-frequency observations are particularly valuable for studying aged electron populations and faint, diffuse radio emission.
6. It connects radio and X-ray astronomy
The radio lobes and the hot intracluster gas influence one another, so observations at different wavelengths are essential for understanding the system.
GMRT and the Search for the Hidden Radio Universe
The significance of the GMRT goes beyond Abell 980.
The telescope was designed specifically for radio astronomy at metre wavelengths and consists of 30 large, steerable 45-metre antennas distributed across a region extending up to about 25 km. NCRA describes GMRT as a major low-frequency radio interferometer used by astronomers around the world.
Its low-frequency capabilities allow astronomers to investigate objects and phenomena that can be difficult to see at higher radio frequencies.
That includes:
- aged radio galaxies,
- radio relics,
- pulsars,
- active galactic nuclei,
- galaxy clusters,
- the interstellar medium,
- and other faint radio phenomena.
The Abell 980 study is therefore an excellent example of how low-frequency radio astronomy can reveal structures that are almost invisible at other wavelengths.
The Abell 980 Discovery in Simple Words
The entire story can be summarised in a few steps:
1. A massive galaxy became active.
Its central supermassive black hole powered powerful relativistic jets.
↓
2. The jets inflated huge radio lobes.
↓
3. The active phase ended.
The old lobes stopped receiving fresh energy and began to fade.
↓
4. The cluster environment helped preserve them.
Hot intracluster gas confined the radio plasma and helped extend its detectable lifetime.
↓
5. The galaxy moved toward the cluster centre.
This movement is part of the researchers' proposed evolutionary model.
↓
6. The central engine became active again.
A new, smaller radio double appeared around the galaxy's newer position.
↓
7. The old and new radio structures became spatially separated.
↓
8. Researchers proposed a Detached-Double-Double Radio Galaxy.
This unusual structure provides a possible record of multiple episodes of supermassive-black-hole jet activity.
Frequently Asked Questions (FAQs)
Q. What did GMRT discover in Abell 980?
GMRT observations helped reveal and characterise two large, diffuse, ultra-steep-spectrum radio sources in the Abell 980 galaxy cluster. They are interpreted as relic radio lobes from an earlier episode of activity associated with the brightest cluster galaxy.
Q. How old are the fossil radio lobes?
Their estimated spectral age is approximately 260 million years, or about 26 crore years.
Q. Is the galaxy itself 260 million years old?
No. The approximately 260-million-year estimate applies to the old radio structures and their associated past jet activity, not the age of the entire galaxy.
Q. What is Abell 980?
Abell 980 is a galaxy cluster containing galaxies, a hot intracluster medium and several radio-emitting structures. It is the environment in which the unusual fossil radio lobes were studied.
Q. Where is GMRT located?
GMRT is located near Khodad, approximately 80 km north of Pune, Maharashtra, and is operated by the National Centre for Radio Astrophysics (NCRA), part of the Tata Institute of Fundamental Research (TIFR).
Q. How many antennas does GMRT have?
GMRT consists of 30 fully steerable antennas, each 45 metres in diameter, distributed over a region extending up to about 25 km.
Q. What is a fossil radio lobe?
A fossil or relic radio lobe is an old radio-emitting structure left behind after the powerful jets that created it have weakened or switched off.
Q. What is a Double-Double Radio Galaxy?
A Double-Double Radio Galaxy is a system in which two pairs of radio lobes are associated with separate episodes of activity from the same central galaxy.
Q. What is a Detached-Double-Double Radio Galaxy?
A dDDRG is a proposed morphological class in which the older and younger radio-lobe pairs of a double-double radio galaxy are spatially offset rather than being normally collinear. Researchers proposed that Abell 980 may be a particularly plausible example.
Q. Which telescopes were used to study Abell 980?
The research used GMRT and EVLA/VLA radio observations along with Chandra X-ray data. The later dDDRG study also used LOFAR LoTSS-2 observations at 144 MHz.
Q. Why are low-frequency radio observations useful?
Older populations of relativistic electrons tend to produce very steep radio spectra. Low-frequency observations can therefore be particularly effective for finding faint, aged radio structures.
Q. When was the GMRT fossil radio-lobe discovery published?
The main research results were published in 2022. The first study appeared in Astronomy & Astrophysics, and the dDDRG interpretation appeared in Publications of the Astronomical Society of Australia.
Conclusion
The GMRT observations of Abell 980 have revealed a remarkable example of how the Universe can preserve evidence of ancient black-hole activity.
The two large diffuse radio structures are estimated to be about 260 million years old and are interpreted as relic radio lobes from an earlier episode of jet activity associated with the brightest galaxy in the cluster.
What makes the system particularly intriguing is the presence of a much younger radio double near the current location of the galaxy. Researchers have proposed that the old and young structures may have been produced during two separate episodes of activity by the same galaxy, with the parent galaxy drifting toward the cluster centre between those episodes.
Because the old and young radio-lobe pairs are laterally displaced, the system has been proposed as a particularly compelling candidate for a Detached-Double-Double Radio Galaxy (dDDRG).
The discovery also demonstrates why low-frequency radio astronomy is so powerful. At first glance, an ancient radio lobe may appear to have disappeared from the Universe. But with sensitive radio observations from facilities such as GMRT and LOFAR, astronomers can recover these faint remnants and use them to reconstruct events that happened hundreds of millions of years ago.
In that sense, the radio sky contains its own form of cosmic archaeology—and Abell 980 offers a particularly fascinating example.
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Scientific Sources & References
1. Salunkhe et al. (2022)
“Deciphering the ultra-steep-spectrum diffuse radio sources discovered in the cool-core cluster Abell 980”
Published in Astronomy & Astrophysics, Volume 664, A186 (2022). The study analysed Chandra X-ray data and GMRT observations at 150 and 325 MHz, together with EVLA observations at 1.5 GHz.
Publication link: https://doi.org/10.1051/0004-6361/202243438 (Salunkhe et al., 2022, A&A, in press)
2. Gopal-Krishna et al. (2022)
“The radio source in Abell 980: A Detached-Double-Double Radio Galaxy?”
Published in Publications of the Astronomical Society of Australia, Volume 39, e049 (2022). The paper combined LoTSS-2, GMRT and VLA observations to strengthen the proposed dDDRG interpretation.
Publication link: https://doi.org/10.48550/arXiv.2207.05166 [10.1017/pasa.2022.30](Gopal-Krishna et al., 2022, PASA, in press)
National Centre for Radio Astrophysics (NCRA-TIFR)
NCRA's official GMRT information confirms that the telescope is located about 80 km north of Pune and consists of 30 fully steerable 45-metre antennas spread across a region of up to 25 km.
The research team:
Dr Surajit Paul, an Astrophysicist from Savitribai Phule Pune University (SPPU),
Sameer Salunkhe is Dr Paul’s PhD student,
Dr Satish Sonkamble is a postdoc at INAF-Padova Astronomical Observatory, Italy,
Shubham Bhagat, (an alumni of Dr Paul’s lab), is a PhD student at TLS, Jena, Germany.
Prof. Gopal Krishna is a Senior Scientist of the Indian National Science Academy, working at UM-DAE Centre for Excellence in Basic Sciences (CEBS), Mumbai.
Contact:
- Surajit Paul (surajit@physics.unipune.ac.in); Mob. 9405510226
- Gopal Krishna (gopaltani@gmail.com); Mob. 9850415719
- Sameer Salunkhe (sameer24.salunkhe@gmail.com); 8668287219
- Satish Sonkamble (satish04apr@gmail.com); 7410749277
- Yashwant Gupta (ygupta@ncra.tifr.res.in) ; Phone: 020-25719242
- Nissim Kanekar (nkanekar@ncra.tifr.res.in); Phone: 020-25719246
- CH. Ishwara-Chandra (ishwar@ncra.tifr.res.in); Mobile: 9403136630
- J. K. Solanki (solanki@ncra.tifr.res.in); Mobile: 9890447888
- Anil Raut: (anil@gmrt.ncra.tifr.res.in); Mobile: 8605525945
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