The night sky is full of rings and bubbles, but in the radio band they look different. In data from the Australian radio telescope ASKAP, a structure has surfaced that resembles two soap bubbles blown from the same straw. One bubble is almost perfectly round, while the second slightly overlaps it, as if they were exhaled a second apart. At the center of this cosmic soapy wonder lies a galaxy that no one would suspect of activity: old, massive, and long settled.
Usually, such giant radio bubbles are the result of shock waves during mergers of galaxy clusters, where hot gas is compressed and accelerates electrons to speeds that make them shine in the radio band. But here the environment is poor: no cluster, no dense medium. Yet the shape (two nearly perfect circles, each with a radius of 114 kiloparsecs, with centers separated by 171 kiloparsecs) is too neat for a chaotic shock wave. Rather, these are traces of an ancient outburst from the galaxy's core, when the supermassive black hole at its center briefly woke up and spat out two opposite jets.
The bubble's spectral index is steep: it shines brighter at low frequencies than at high ones. This is like an old campfire: the flames are gone, but the embers still glow. The electrons emitting radio waves have lost energy, meaning the outburst happened not yesterday but hundreds of millions of years ago. Moreover, toward the bubble's edge, the spectrum becomes flatter: there, electrons may be re-accelerated, compressed by a residual shock wave.
The host galaxy LEDA 217397 is a typical "sleeping giant": its mass is about 10 billion solar masses, it forms almost no stars, and no active nucleus is visible in the mid-infrared. But it is precisely such galaxies that sometimes preserve the memory of a turbulent youth. A merger of two supermassive black holes could have briefly switched on the jet mechanism, and then everything quieted down again — leaving only two bubbles slowly expanding into intergalactic space.
This discovery forces a fresh look at the mysterious "odd radio circles"—ring-like structures whose nature astronomers have been unable to explain for several years. Some of them may actually be double bubbles that appeared as single rings at low resolution. If so, sleeping galaxies are not so sleeping after all: from time to time they blow giant soap bubbles into space, and we are only beginning to learn to notice them.
The bubble's diameter is about 400,000 light-years, four times the diameter of our Galaxy. Yet its radio luminosity is comparable to that of an ordinary radio galaxy, and on the sky it covers an area dozens of times larger than the disk of the Moon.
In Stanisław Lem's novel Fiasco, giant bubbles left by the outflows of extraterrestrial civilizations are described. Here, a similar trace was likely left by a black hole at the center of a galaxy, but without any intelligent purpose.