Astronomers using NASA's Chandra X-ray Observatory have traced the likely origin of a radio-wave formation nicknamed the "Treble Clef" for its resemblance to the musical symbol. A paper led by Andrea Botteon of the National Institute for Astrophysics in Italy was published Wednesday in the Astronomy & Astrophysics journal.

The shape was hard to explain because it lies almost directly behind the plane of the Milky Way, whose stars, gas and dust largely block the view from Earth. Using Chandra and the German eROSITA X-ray telescope, researchers found diffuse hot gas at the location, showing a galaxy cluster hosts the object, and determined the cluster is actually two clusters merging. The team concluded a two-stage process likely formed the Treble Clef: an earlier outburst from a supermassive black hole drove charged particles and magnetic fields outward at close to the speed of light, producing bright radio waves that faded, and the cluster collision then compressed or reenergized the cloud, making it brighter again. Shock waves from the collision may have contributed. The researchers think the direct collision occurred a few hundred million years ago, and the clusters are now moving apart.