An international team co-led by Dr Pratik Dabhade of the National Centre for Nuclear Research (NCBJ) has discovered a remarkable bow-and-arrow-shaped radio galaxy with a giant arc-like structure extending nearly 1.8 million light-years.

The source, named RAD-BAARG, is described in a paper accepted for publication in Monthly Notices of the Royal Astronomical Society: Letters. Its highly asymmetric structure may provide one of the clearest radio signatures yet of a giant bow shock associated with a galaxy moving supersonically through a cluster environment.

The discovery was made using sensitive low-frequency images from the Low-Frequency Array (LOFAR), a pan-European world-class radio telescope, and involved researchers from NCBJ and the RAD@home Astronomy Collaboratory, a citizen-science research initiative in India.

Co-lead author Dr Pratik Dabhade, from the National Centre for Nuclear Research in Poland, said “BAARG is exciting not just because of its striking bow-and-arrow shape, but because it sits in a complex multi-halo environment where gas flows, infall, and possible shocks can reshape radio plasma. LOFAR allows us to see this faint, low-surface-brightness emission in remarkable detail. With LoTSS DR3 and the future Square Kilometre Array Observatory (SKAO), we may find many more systems where radio galaxies reveal otherwise invisible interactions between jets, galaxies, and their environments.”

A galaxy shaped by its environment

Radio galaxies are powered by supermassive black holes that launch oppositely directed jets of relativistic, magnetised plasma into intergalactic space.

In RAD-BAARG, the western jet feeds a sector-shaped emission region connected to a giant arc spanning almost 560 kiloparsecs, or 1.8 million light-years. On the opposite side, the jet develops a distorted S-shaped structure followed by a faint offset tail extending nearly 600 kiloparsecs.

“The structure of this source is unlike that of any radio galaxy I have seen in the last 25 years,” said lead author Dr Ananda Hota, Founder, Director and Principal Investigator of RAD@home Astronomy Collaboratory. He further added, “Its remarkable morphology appears to display signatures of interaction between relativistic radio plasma and a large-scale shock generated during the galaxy’s infall into a nearby cluster environment.”

The proposed scenario is similar to the shockwave produced ahead of a supersonic aircraft. A galaxy moving faster than the local speed of sound through hot intracluster gas can compress the surrounding medium and generate a large-scale bow shock.

Such shocks have long been predicted by theoretical models and simulations, but detecting them is extremely difficult because the surrounding gas is highly diffuse. The researchers suggest that radio plasma supplied by the galaxy’s jet may be illuminating this otherwise almost invisible structure. Further radio and X-ray observations will be needed to test this scenario conclusively.

A complex multi-halo environment

RAD-BAARG lies in a complex environment containing a confirmed galaxy group and several nearby cluster-scale systems. Gas flows and the possible infall of the host galaxy towards a nearby cluster may be reshaping its radio plasma, making the source a valuable tracer of interactions between galaxies, jets and hot cluster gas.

A timely discovery for LoTSS DR3

The source was identified in the LOFAR Two-metre Sky Survey Data Release 2 (LoTSS DR2), which is particularly sensitive to faint and extended radio emission.

The discovery is especially timely because LoTSS Data Release 3, released earlier this year, now makes available a vastly larger dataset for finding similar systems across the northern sky. Covering more than 80 per cent of the northern sky, LoTSS DR3 is the deepest low-frequency radio survey ever produced over such an enormous area of the sky.

RAD-BAARG provides researchers and citizen scientists with a striking example of the rare and unexpected phenomena that may be uncovered within this vast new dataset. Similar sources may already be hidden among the millions of radio detections. Discoveries of such rare systems can also provide valuable training samples for artificial-intelligence and machine-learning tools designed to carry out automated searches across LoTSS DR3 and future radio surveys.

The ongoing LOFAR2.0 upgrade will further improve LOFAR’s capabilities, while the future Square Kilometre Array Observatory will reveal still fainter and more complex interactions between radio galaxies and their environments.

Following another unusual LOFAR discovery

The RAD-BAARG discovery follows the team’s 2025 identification of the most distant and most powerful Odd Radio Circle known at the time. That source was also discovered by researchers associated with NCBJ and RAD@home using LOFAR observations.

Read the earlier NCBJ report here: https://www.ncbj.gov.pl/en/news/discovery-most-powerful-odd-radio-circle-date 

Re:search Paper link https://doi.org/10.1093/mnras/stag1033

Royal Astronomical Society press release link: https://ras.ac.uk/news-and-press/research-highlights/bow-and-arrow-shaped-radio-galaxy-discovered-citizen-scientist

RAD@home Astronomy Collaboratory: https://radathomeindia.org/

LOFAR Surveys: https://lofar-surveys.org/

Science contacts: 

Dr Pratik Dabhade
Astrophysics Division, National Centre for Nuclear Research, Warsaw, Poland
Email: pratik.dabhade@ncbj.gov.pl

Dr Ananda Hota 
UM-DAE Centre for Excellence in Basic Sciences; CETACS, University of Mumbai; RAD@home Astronomy Collaboratory, India
Email: hotaananda@gmail.com