Chandra Uncovers How a Galaxy Cluster Collision Formed the Treble Clef Shape

Astronomers recently trained NASA’s Chandra X-ray Observatory on a strange radio formation that sits behind the dense band of stars, gas, and dust along the Milky Way’s plane. From our vantage the structure looks almost exactly like the musical symbol known as a treble clef, a resemblance that led researchers to call the whole scene the Treble Clef. Chandra and Germany’s eROSITA telescope both recorded the presence of gas heated to millions of degrees, the clear signature of a galaxy cluster, one of the largest objects in the universe. Further scrutiny of that gas showed the cluster is actually two separate clusters that ran into each other.

A composite image combines all of the observations in one place. The blue areas represent the heated gas that Chandra and eROSITA mapped, while the red bits represent the radio waves detected by the LOFAR array, and if you look closely, you’ll notice the vertical lines that form the Treble Clef pattern. The stars and galaxies around it are illuminated in white by the UKIDSS-GPS survey. The radio structure is quite large, spanning millions of light-years and covering two clusters, each with hundreds of galaxies and enough superheated gas to blow your socks off.

The treble clef shape evolved at several stages. First, a supermassive black hole at the center of one of the clusters decided to have a bit of a blow out, sending clouds of charged particles and magnetic fields flying through space at nearly the speed of light. These particles emitted a slew of brilliant radio waves, which faded away as they lost energy. The two groups eventually collided. The hit squeezed the dying radio cloud, awakening it all over again. The shock waves from the collision most likely contributed to the current pattern, similar to the sonic booms that follow a plane taking off.

By carefully removing the smooth, oval shape of the hot gas from the Chandra data, the researchers confirmed the collision occurred. What remained was a picture that was brightest on the left and right sides, indicating that the clusters arrived along those sides. It all happened a few hundred million years ago, and since then, the clusters have been gradually drifting apart. The Treble Clef is simply what humans do when we see familiar forms in random patterns that have no real connection to music. Some perceive it as a dragon, while others view it as the same thing; either way, this image captures one of the rare instances when a black hole outburst and a cluster collision combine to create a permanent impression across millions of light-years.