NASA Telescopes Trace a Ghostly Radio 'Treble Clef' to a Galaxy-Cluster Collision Hidden Behind the Milky Way

Astronomers have traced a wispy, musically shaped filament of radio waves to its true source: two enormous galaxy clusters colliding almost directly behind the plane of the Milky Way, in a patch of sky so choked with the galaxy's own dust and stars that the collision went unrecognized for more than two decades. A composite image released October 14 by NASA's Chandra X-ray Center combines X-ray, radio and infrared data to show, for the first time, hot gas confirming that the radio structure — nicknamed the "Treble Clef" for its looping shape — sits inside a massive cluster merger roughly 1.55 billion light-years from Earth.

The findings, from a team led by astronomer Andrea Botteon of Italy's National Institute for Astrophysics, are described in a paper accepted for publication in Astronomy & Astrophysics. Using NASA's Chandra X-ray Observatory together with the Russian-German eROSITA telescope, the researchers measured the cluster's redshift at 0.117, placing it well beyond earlier, rougher estimates. Its hot intracluster gas glows at a temperature of roughly 6 kiloelectronvolts — tens of millions of degrees — and the system's total mass works out to about 5.4 to 6.0 quadrillion times the mass of the sun, typical for a large merging cluster. X-ray data also show the telltale lopsidedness of gas that has not yet settled into equilibrium, the signature of an ongoing, violent merger rather than a single relaxed cluster.

The radio filament itself stretches across roughly 760,000 parsecs — about 2.5 million light-years — making it one of the largest, steepest-spectrum structures of its kind ever cataloged. A shorter companion filament runs about 180,000 parsecs, or nearly 590,000 light-years, and the team also found a fainter haze of diffuse radio emission near the cluster's center that may be a separate, related structure called a radio halo.

A galaxy cluster hiding behind the Milky Way

The object had been noticed before, but misunderstood. It first showed up in older radio surveys at a galactic latitude of roughly half a degree — essentially skimming the disk of our own galaxy as seen from Earth. That position places it inside what astronomers call the Zone of Avoidance, a band circling the sky where the Milky Way's gas, dust and crowded starfields absorb and scatter light from anything farther away. Earlier observers first mistook the signal for a pulsar, a rapidly spinning dead star, before a 2004 study proposed it might instead belong to a galaxy cluster obscured behind the Galactic plane. Confirming that hunch took another two decades, because optical telescopes — the usual way astronomers measure a cluster's distance and mass — are largely useless when the Milky Way itself is in the way.

X-ray light cuts through that obscuring dust far more easily than visible light, which is what let Chandra and eROSITA succeed where earlier attempts stalled. The telescopes detected multimillion-degree gas exactly where the radio filament was located, tracing the shape of a cluster caught mid-collision. The researchers subtracted a smooth, symmetric model of the gas from the X-ray image and found a residual pattern consistent with two clusters that collided along one axis a few hundred million years ago and are now moving apart again.

How the picture came together

The team's reconstruction leans on a two-act explanation for why the radio emission looks the way it does. In the first act, long before the merger, a supermassive black hole at the center of a galaxy in the cluster unleashed an outburst, firing jets of charged particles and magnetic field out into the surrounding gas. That plasma glowed brightly in radio waves for a time, then faded as its electrons lost energy — becoming, in effect, a radio fossil. In the second act, the arrival of the colliding cluster's shock waves and turbulent gas motions swept up that fossil plasma, compressing it and reviving the radio glow in a new, filamentary shape. Astronomers call this kind of revived source a radio phoenix, after the mythological bird reborn from its own ashes.

To pin down the physics, the researchers combined new observations from India's upgraded Giant Metrewave Radio Telescope with archival data from the Netherlands-based LOFAR low-frequency array, mapping how the radio brightness changes across frequencies. A spectrum that steepens sharply at higher frequencies is a hallmark of aging plasma, and the Treble Clef's spectrum does exactly that — consistent with electrons accelerated long ago that have been losing energy ever since, only to be freshly stirred by the merger.

• Redshift z = 0.117, placing the cluster roughly 1.55 billion light-years from Earth
• Cluster mass: approximately 5.4–6.0 × 10^14 solar masses, with a gas temperature near 6 kiloelectronvolts
• Radio filament length: about 760,000 parsecs (2.5 million light-years), among the largest "radio phoenix" structures known
• Merger timing: a collision roughly a few hundred million years ago; the two clusters are now moving apart

Why a hidden collision matters

The discovery matters less for any single cluster than for what it implies about an entire, undercounted population. Large sky surveys used to build cosmological maps of galaxy clusters routinely exclude the Zone of Avoidance, which covers a meaningful fraction of the sky, because optical and infrared cluster-finding methods fail there. If a cluster this massive — several quadrillion solar masses, among the largest gravitationally bound structures in the universe — could go unrecognized for 20 years in plain sight, current catalogs of the universe's largest structures are likely missing entries, with consequences for studies that use cluster counts to measure the universe's expansion history and the growth of cosmic structure over time.

The find also adds a well-measured example to a short list of radio phoenixes found at the intersection of cluster mergers and ancient black hole activity, giving astrophysicists another data point for modeling how shock waves re-accelerate aging plasma throughout the universe's largest structures.

"We conclude that this source is a radio phoenix belonging to a massive, merging galaxy cluster in the Zone of Avoidance," the researchers write in their paper, adding that the structure's complex shape was "likely responsible for the generation of the candidate radio halo tentatively observed in the cluster center."

What happens next

The team flags some findings as provisional — the faint candidate radio halo needs confirmation, and the merger's geometry is inferred rather than directly observed. The authors point to the planned LOFAR2.0 upgrade, which will sharpen the array's resolution at very low frequencies, as the logical next step for resolving the filament in more detail and testing whether the halo is real. Broader follow-up surveys of the Zone of Avoidance using X-ray telescopes, far less hampered by Galactic dust than optical instruments, are likely to turn up more clusters that optical surveys have missed — a prospect that could modestly revise astronomers' current tally of the universe's largest structures.

Chandra, the X-ray observatory at the center of the find, has now been in operation for more than a quarter-century, managed for NASA by the Marshall Space Flight Center with science operations run by the Smithsonian Astrophysical Observatory's Chandra X-ray Center in Cambridge, Massachusetts. Its continued ability to resolve faint, hot gas in crowded, dust-choked regions of sky — exactly the kind of place this cluster was hiding — underscores why NASA has pushed to keep the telescope, and partner instruments like eROSITA, collecting data even as newer observatories come online.