An Indian citizen scientist working with the RAD@home network identified a radio galaxy with a unique bow-and-arrow shape in LOFAR survey data - the clearest radio signature yet of a galaxy falling supersonically into a galaxy cluster.
A citizen scientist working with India’s RAD@home astronomy network has identified a radio galaxy with a shape unlike any previously recorded: a distinct bow-and-arrow structure with an arc spanning roughly 1.8 million light-years. The discovery, published in Monthly Notices of the Royal Astronomical Society: Letters, represents the clearest radio-wavelength signature yet of a galaxy falling supersonically into a neighbouring cluster environment and generating a large-scale bow shock.
The galaxy, designated RAD J104501.6+352852 and nicknamed BAARG (Bow-And-Arrow Radio Galaxy), sits at a redshift of z = 0.159 - approximately 2 billion light-years from Earth. It was spotted not by a trained astronomer but by a participant in a weekend online class run by RAD@home, a citizen science network founded in India that analyses images from major radio surveys and regularly publishes results in peer-reviewed journals.
Radio galaxies are powered by supermassive black holes at their centres. Energetic jets of plasma eject outward from the nucleus, often stretching for millions of light-years and glowing brightly at radio wavelengths. In BAARG, those jets have been sculpted into a highly asymmetric pattern.
On its western side, the jet feeds into a broad, sector-shaped emission region that curves backward into a 560-kiloparsec arc - roughly 1.8 million light-years - the “bow.” On the eastern side, the jet bends into an S-shaped distortion before fading into a fainter, offset tail extending approximately 600 kiloparsecs - the “arrow.” The full extent of both lobes spans several million light-years from end to end.
The team, led by researchers Hota, Dabhade, and Ghosh and collaborators at the RAD@home Astronomy Collaboratory, proposed that the bow-shaped arc traces a large-scale shock front. According to the paper, the host galaxy appears to be falling supersonically toward a nearby cluster of galaxies. As it moves through the hot intracluster medium - the diffuse gas filling the space between cluster galaxies - the galaxy’s own circumgalactic medium and radio jets are compressed and swept backward. The result, in this interpretation, is a vast arc of compressed radio-emitting plasma marking the leading edge of the shock, analogous to the bow wave ahead of a supersonic aircraft.
The morphology itself is documented in data from the LOFAR Two-metre Sky Survey (LoTSS DR2), one of the deepest low-frequency radio surveys yet conducted, and confirmed in the published paper. The bow-shock interpretation is the team’s leading explanation and is described as consistent with the observations.
However, the authors note that alternative explanations - including environmental pressure gradients or projection effects - cannot yet be fully excluded. No direct velocity measurement confirming supersonic infall has been published; that inference is drawn from the galaxy’s morphology and the structure of the surrounding multi-halo cluster environment. Additional observations would be needed to constrain the scenario further.
BAARG resides in a dynamically complex region with several cluster-scale systems at similar redshifts, a setting the paper describes as a “multi-halo environment.” This context is central to the bow-shock interpretation: the gravitational dynamics of multiple overlapping halos are thought to be driving the host galaxy’s infall trajectory.
The discovery was made visually, not by an automated detection system. The citizen scientist identified the unusual bow-and-arrow shape while reviewing LOFAR images during a RAD@home weekend session. According to the Royal Astronomical Society, which highlighted the result, the morphology is the kind of visually complex, asymmetric structure that machine classifiers regularly miss or assign to noise.
RAD@home, which operates as an open citizen science collaboratory for radio astronomy in India, has previously co-authored results in peer-reviewed journals and has a track record of identifying anomalous sources in survey data that fall outside standard templates. The network works with professional astronomers who then carry out follow-up analysis and lead the formal publication process.
Galaxy clusters grow by accreting smaller groups over cosmic time. Models of this process predict that infalling galaxies will generate bow shocks as they push through the cluster’s hot gas at supersonic speeds. Observing those shocks directly - and in detail - has been difficult. Radio jets, because they are sensitive to the pressure of surrounding gas, can in principle trace these shock structures. BAARG, if the bow-shock interpretation is confirmed, would be among the most detailed radio-wavelength tracers of this process identified to date.
According to the Royal Astronomical Society, the research team intends to search the broader LoTSS dataset for additional bow-shock candidates with similar morphologies, potentially building a sample large enough to study the phenomenon statistically.
The paper, “RAD@home discovery of a bow-and-arrow radio galaxy tracing a ~560 kpc bow-shock structure in a multi-halo environment,” is published in Monthly Notices of the Royal Astronomical Society: Letters and is available as a preprint at arXiv:2606.23106.
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