The galaxy cluster RXC J0528.9−3927 is a true metropolis of hundreds of galaxies, immersed in hot gas. At its center shines a bright radio halo — a cloud of emission produced by fast electrons spiraling in magnetic fields. But the MeerKAT telescope has noticed an additional, much fainter glow surrounding this halo. It extends over two million light-years — like a fog enveloping the entire city.
To discern this cloud, astronomers collected light from the cluster for 12.9 hours, then subtracted the emission from all the bright galaxies. What remained was only faint diffuse glow. It turned out that its brightness fades more slowly than it would if it were just the central halo. This implies there is an additional source outside — a megahalo candidate. This is the first such discovery at gigahertz frequencies. Now the scientists plan new observations to confirm the finding and to understand how common such clouds are in the universe.
Megahalos occupy volumes up to 30 times larger than typical radio halos, yet their glow is 20 times fainter, like a giant ghostly bubble around a bright core.
In Stanisław Lem's novel 'Solaris', the planet's ocean creates giant structures invisible in ordinary light — somewhat like megahalos, which astronomers can only discern by their faint radio glow.