Treble Clef, a 2-million-light-year cosmic structure seen in X-rays

Hidden behind our own Milky Way, a radio structure stretches over 2 million light-years and, seen from Earth, looks like a treble clef. Its visible light is blocked by the galaxy we live in, so astronomers read it through radio waves, which pass through the dust that obscures the stars.

The object was examined in 2004, catalogued as VLSS J0318.9+5755 and nicknamed Treble Clef, and for years its nature remained unknown. Recent observations by the Chandra X-ray Observatory and the eROSITA instrument changed that picture. The X-ray data, shown in blue in the composite image, revealed a new layer of the structure that radio alone could not show.

The working hypothesis is a chain of events. A supermassive black hole at the center of a galaxy would have expelled a cloud of charged particles. Once the radio emission from those particles faded, a shock wave produced by the collision of two clusters of galaxies may have reinvigorated them, lighting the structure up again. The radio emission appears in red in the composite image, and the X-rays in blue.

So what does this change? For astronomers, objects like Treble Clef are rare and useful: they offer clues about how entire clusters of galaxies interact, and about the broader evolution of the universe. A single hypothesis does not settle the question, and the observations described so far do not yet confirm every step of the sequence. But the object now has a plausible story, where for more than twenty years it had none.

The image credits name the teams behind the work: the Chandra X-ray data credited to A. Botteon and colleagues, with radio data from the ASTRON LOFAR array and infrared data from the UKIDSS survey. Each instrument sees a different layer of the same giant structure, and the combination is what makes the hypothesis possible.