Bow-and-arrow-shaped radio galaxy discovered by citizen scientist

Astronomers have discovered a “remarkable” bow-and-arrow-shaped radio galaxy with an enormous arc-like structure extending nearly 1.8 million light-years across. The newly identified system, detailed in a new paper published today in Monthly Notices of the Royal Astronomical Society: Letters, has a “highly unusual” and asymmetric structure unlike those seen in standard radio galaxies.

It was detected by an international team of researchers working with RAD@home Astronomy Collaboratory for citizen science research in India, using ultrasensitive images from the Low-Frequency Array (LOFAR) radio telescope. They say it may represent one of the clearest known radio signatures of a giant bow shock generated by a galaxy falling supersonically into a cluster environment.

“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. “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 discovery of the source—named RAD-BAARG (Bow-And-Arrow Radio Galaxy)—was made using data from the LOFAR Two-meter Sky Survey (LoTSS), one of the deepest radio surveys ever conducted at low frequencies.

Radio galaxies are powered by supermassive black holes located at the centers of galaxies, which launch enormous jets of relativistic magnetized plasma into intergalactic space.

A bow shock lit by radio jets

In RAD-BAARG, the researchers say one of the jets appears to interact with a large bow shock-like structure formed as the host galaxy falls through the surrounding hot gas toward a nearby cluster of galaxies.

Similar to the shock wave formed ahead of a supersonic aircraft, a galaxy moving faster than the speed of sound in the surrounding intracluster medium can compress the ambient gas and generate a large-scale shock front.

The radio-emitting plasma from RAD-BAARG appears to illuminate this otherwise extremely faint structure, making it visible in low-frequency radio images, according to the team. The western side of the source contains a narrow jet feeding a sector-shaped emission region and a giant arc-like feature extending over nearly 560 kiloparsecs (1.8 million light-years).

On the opposite side, the jet develops a distorted S-shaped morphology, followed by a faint offset tail extending to almost 600 kiloparsecs. The overall structure suggests strong interaction between the radio plasma and the surrounding large-scale environment.

A crowded and turbulent setting

The research team found that the host galaxy resides within a dynamically complex environment containing nearby cluster-scale systems at similar distances.

The observed morphology is consistent with interaction between the radio jets and large-scale environmental gradients, bulk gas motions and possible shock-related compression associated with the galaxy’s infall.

Although theoretical studies and computer simulations have long predicted bow shocks around infalling galaxies, detecting them directly has proven extremely difficult because the surrounding gas is extraordinarily diffuse and faint.

A few candidate systems have previously been hinted at in X-ray observations, but RAD-BAARG provides an unusually detailed radio view of such a phenomenon.

Co-lead author Dr. Pratik Dabhade, from the National Center 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 Kilometer Array Observatory (SKAO), we may find many more systems where radio galaxies reveal otherwise invisible interactions between jets, galaxies and their environments.”

Another lead author, Dr. Shubhrangshu Ghosh, of SRM University Sikkim in India, said, “The reported observation reveals the first direct imaging of characteristic arc-shape morphology in radio frequency in regard to supersonically infalling radio-galaxy (most likely) onto a cluster medium—a spectacular textbook example of a large bow shock.

“Discovery of more such sources and their study during the SKAO era will provide much deeper insight about jet-ambient medium interaction and consequent feedback processes.”

Citizen science and the next surveys

The unusual source was initially noticed by RAD@home citizen scientist Pranim Limbo while inspecting LOFAR survey images. Coming from a remote Himalayan hill region and without access to a major astronomy institute, the discovery highlights the power of collaboratory-style citizen science research in enabling university students and motivated learners to take part in frontline astronomical research.

Since 2013, RAD@home has trained participants across India to analyze astronomical data from world-class telescopes and contribute to professional scientific discoveries irrespective of their geographic or institutional backgrounds.

The discovery also points to future possibilities for next-generation radio astronomy facilities such as the SKAO, which is currently under construction and expected to become the world’s most powerful radio telescope.

Future ultrasensitive surveys may uncover many more examples of shock-related interactions around infalling galaxies and help astronomers better understand how radio galaxies evolve within the large-scale cosmic environment.

The team is also hoping that artificial intelligence and machine-learning techniques could be used to identify additional unusual radio galaxies hidden within the enormous data volumes expected from upcoming radio sky surveys.

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Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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