Subtract the Smooth Part of a Galaxy Cluster, and the Collision Appears

The most informative X-ray view of the galaxy cluster behind the “Treble Clef” radio source is partly an image of what was left after astronomers removed a smooth oval.

That sounds like image manipulation because it is image manipulation, in the literal and useful scientific sense. The researchers had X-ray observations of very hot gas spread through the cluster. Most of that emission was roughly symmetrical. They modeled the broad oval-shaped component, subtracted it from the original Chandra image, and examined the residuals.

The leftovers were not symmetrical. X-ray emission was brighter to the left and right of the cluster center and fainter through the middle. That pattern helped establish the direction of a collision between two galaxy clusters.

The system is about 1.55 billion light-years away and lies almost directly behind the plane of the Milky Way, where foreground stars, gas, and dust make ordinary optical observation difficult. The team assembled it from evidence collected in different parts of the spectrum: X-rays from Chandra and eROSITA, radio observations from LOFAR and other facilities, and infrared observations from UKIRT.

The shape that earned the “Treble Clef” nickname appears in the radio data. The merger geometry came from a different measurement.

Remove the oval and inspect what remains

Most of the cluster's hot X-ray-emitting gas can be approximated by a smooth oval. The researchers fitted that large-scale component and removed it from the Chandra image.

If the cluster really behaved like the simple model, the residual image would contain little coherent structure beyond noise. Instead, it showed excess X-ray emission on opposite sides of the center.

Botteon and colleagues interpret that left-right asymmetry as evidence that two clusters passed through one another along roughly the same axis a few hundred million years ago and are now separating.

The residual image is useful precisely because the model is deliberately incomplete. It supplies a baseline. Where the real cluster departs from that baseline, structure that was difficult to see against the bright overall emission becomes obvious.

Nothing about the subtraction makes the collision appear from nowhere. The original photons are still the measurements. The model gives the researchers a way to ask which parts of their spatial distribution are ordinary enough to ignore for this question and which parts require an explanation.

The radio source is older than the collision

The cluster merger accounts for only part of the Treble Clef's history.

The researchers identify the radio source as a radio phoenix, fossil plasma from an older period of activity around a supermassive black hole that later brightened again. In the proposed sequence, the black hole first expelled energetic particles and magnetic fields into the cluster environment. Their radio emission faded as the particles aged.

The later collision compressed or reenergized that old plasma, possibly through shocks associated with the merger. At radio frequencies the source is extraordinarily steep-spectrum, reaching a spectral index greater than 4 between 400 and 650 MHz. That makes it especially bright toward lower frequencies and is one reason the team points to future very-low-frequency observations as useful.

The different telescopes are answering different questions. X-rays trace the multimillion-degree gas and the merger. Radio data expose the old nonthermal plasma. Infrared observations help identify galaxies in a region obscured by the Milky Way foreground.

NASA's Chandra team calls the treble-clef resemblance pareidolia. A sideways dragon works too. Neither resemblance carries physical information.

The system remains an awkward target because of its position behind the Galactic plane. Now, at least, follow-up observers know they are looking at a merging cluster with an aged radio source that has been reenergized, and they have a specific prediction about where additional low-frequency radio data should be most informative.

Sources

• Andrea Botteon et al., “The Treble Clef radio phoenix and its old nonthermal filaments,” Astronomy & Astrophysics (2026; preprint available): https://arxiv.org/abs/2607.06659
• Chandra X-ray Center, “NASA's Chandra Breaks Down 'Treble Clef's' Galaxy Cluster Collision” (October 14, 2026): https://chandra.harvard.edu/photo/2026/trebleclef/
• NASA Science, “NASA's Chandra Breaks Down 'Treble Clef's' Galaxy Cluster Collision” (October 14, 2026): https://science.nasa.gov/missions/chandra/nasas-chandra-breaks-down-treble-clefs-galaxy-cluster-collision/