In the Coma galaxy cluster, about 320 million light-years away, an invisible cloud stretches out. It shines in the radio band: this is a radio halo, a giant collection of charged particles accelerated to nearly the speed of light. Astronomers have long observed it, but at a frequency of 4.8 gigahertz, the signal had remained elusive: it was drowned out by interference from terrestrial transmitters and the emission of distant galaxies themselves.
Now a team led by Rashid Sunyaev used the Sardinia Radio Telescope — a 64-meter dish nestled in the mountains of Sardinia. After painstakingly cleaning the data of radio interference and the contribution of individual galaxies, and correcting for the Sunyaev–Zeldovich effect (where the cluster's hot gas slightly distorts the cosmic microwave background), the signal emerged with a confidence above three sigma. The flux was 61±11 millijanskys — equivalent to a light bulb of a few milliwatts placed at the distance of the Moon, smeared over the area of an entire constellation.
Comparing the new result with data at 6.6 gigahertz, the scientists derived a spectral index α≈1.17 (where the flux falls with frequency as ν^(-α)). This means that between 4.8 and 6.6 GHz, the emission fades more slowly than between 1.4 and 4.8 GHz. Such behavior resembles turbulent re-acceleration of particles, but with one condition: the initial electrons must have a spectrum extending to very high energies. Where could they come from? One possibility is a continuous supply from the annihilation of dark matter particles. If so, the radio halo could become an indirect detector of the invisible substance that makes up a quarter of the universe.
Thus, the cloud in the Coma cluster, once considered mere noise, turns into a natural laboratory. It tells us about magnetic fields, turbulence, and possibly the nature of dark matter. The next step is observations at even higher frequencies to understand where exactly the spectrum breaks and what feeds this giant cosmic halo.
The radio halo in the Coma cluster is so vast that light takes several million years to cross it from one end to the other.