The galaxy cluster RXC J0528.9−3927 is like a city shrouded in fog: at its center, a radio halo shines brightly, but farther out, beyond the usual boundaries, astronomers have spotted a barely visible glow. It stretches for two million light-years — three times farther than the halo itself. The MeerKAT telescope in South Africa gathered this faint light for 12.9 hours, like a photographer capturing the night sky with a long exposure.
To see this ghostly cloud, astronomers had to subtract the light from all bright galaxies and quasars, much like a restorer removing later overpaint to reveal an old fresco. The remaining glow was compared with a model: if it were only the central halo, the brightness should have faded smoothly toward the edge. But at about 700,000 light-years from the center, the decline slowed, as if beyond the city, a different, more tenuous fog began.
This result is like the first photograph of a rare beast: now it's clear that megahaloes exist at gigahertz frequencies too, but confirming the find will require new observations at different frequencies. If the spectrum of the outer cloud turns out to be steeper than that of the central halo, there will be no doubt. Then astronomers will begin a systematic search for such ghosts across the sky, and we'll learn how common they are and what role they play in the lives of clusters.
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.