Astronomers peering past the crowded, dusty plane of the Milky Way have spotted a tangle of radio light, glowing inside a massive collision of lots of galaxies.
The space object loops and curls in thin strands, earning its nickname, the Treble Clef. It's what researchers refer to as a "radio phoenix," a cloud of faded particles brought back to life. It lurks inside a galaxy cluster — a huge group of galaxies bound together by gravity — and that cluster is in the throes of a merger with another cluster.
Long ago, a giant black hole in the cluster fired out jets of energetic particles. Over time, those particles lost energy and went dark. Then, the violent crash squeezed and stirred the leftover material, and it lit up again in radio light.
The discovery shows how far radio astronomy has come, according to the researchers, whose paper appears in the Astronomy & Astrophysics journal. A new generation of telescopes tuned to long radio waves, especially the European Low Frequency Array (LOFAR) network, now captures images sharp and sensitive enough to resolve blurry smudges into detailed filament tapestries. Radio telescopes also "see" through the gas and dust that block ordinary telescopes.
The Treble Clef first surfaced in radio surveys years ago, and some astronomers thought it might be a pulsar, a fast-spinning dead star. A 2004 study tied it to a possible galaxy cluster, but still couldn't make out its shape because the telescopes of the time couldn't pick out fine enough detail.
After that, the object faded from view in the scientific literature too. It popped up once more, in a 2022 list of possible "X-shaped" radio sources.
But that changed when the team combed through new LOFAR survey data and saw its twisting structure clearly. They followed up with India's Giant Metrewave Radio Telescope to study the object across a wider range of wavelengths.
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To size up the cluster itself, the team turned to NASA's Chandra X-ray Observatory and the German eROSITA space telescope, which detected X-rays pouring off super-hot gas that fills the cluster. That signal let the researchers pin down the cluster's distance — about 1.6 billion light-years away — temperature, and mass for the first time, and it confirmed the ongoing collision. The cluster weighs about 600 trillion times as much as our sun.
The researchers think churning gas from the smashup mangled the phoenix into its treble clef shape. The same event also may have sparked a broad, dim haze of radio light at the cluster's center, called a radio halo, but so far only the Indian telescope has detected it.
The object ranks among the most extreme faint, spread-out radio glows ever found in the hot gas that fills the space between a cluster's galaxies. These delicate threads could help crack a bigger puzzle about how cosmic rays — high-energy particles zipping through space — travel through clusters. Scientists still debate what keeps them shining.