Bow-and-arrow radio galaxy RAD-BAARG caught falling through cluster at supersonic speed, 1.8 million light-year arc
A galaxy shaped like a bow and arrow has been caught tearing through a distant cluster, its radio arc stretching nearly 1.8 million light-years. Astronomers with the RAD@home citizen-science project spotted it in LOFAR telescope data and estimate it is falling inward at 1,130 to
Astronomers have spotted something that does not fit the textbook: a galaxy shaped like a bow and arrow, racing through a distant galaxy cluster and dragging a glowing arc of radio plasma nearly 1.8 million light-years across. The object, nicknamed RAD-BAARG, was first noticed by volunteers working with the RAD@home Astronomy Collaboratory, a citizen-science program in India, who were combing through ultra-sensitive images from the LOFAR radio telescope. On one side of the galaxy a narrow jet feeds a sweeping arc; on the other, the jet twists into an S-shape trailing a faint tail. That lopsided structure is unlike the symmetric shapes seen in most radio galaxies. The arc appears to be a bow shock, the cosmic equivalent of the wave that piles up in front of a moving boat. As the galaxy falls into the cluster it slams into the thin, hot gas that fills the space between galaxies, and that gas piles up and glows. By measuring the angle of the arc, the researchers estimated the galaxy is falling inward at somewhere between 1,130 and 3,580 kilometers per second, faster than the speed of sound in that gas. Clear, measurable bow shocks like this are rare, and they give astronomers a direct way to study how galaxies interact with the gas in clusters, a process that shapes how galaxies grow and eventually stop forming stars. The find also shows how much volunteer astronomers can contribute, since it began with a single person reviewing telescope data. The study by Hota, Dabhade, Ghosh and colleagues is published in Monthly Notices of the Royal Astronomical Society: Letters.