If our eyes could see radio waves, we might spot giant soap bubbles in the sky. Astronomers have found one such pair: two bubbles, blown from a single straw, hanging around a distant galaxy. The galaxy itself is old and quiet, but something once erupted from its center — leaving these bubbles behind.
Why do astronomers think these bubbles are the result of an outburst, not something else? Galaxies harbor supermassive black holes at their cores. Occasionally, a black hole begins to swallow gas, and twin jets shoot out in opposite directions. These jets inflate the surrounding gas, much like a straw inflates a soap bubble. Eventually, the black hole calms down, the jets vanish, but the bubbles remain — slowly expanding for millions of years.
Previously, such bubbles were mostly found in galaxy clusters, where hot gas is abundant. But here, the galaxy is isolated, with a sparse environment — and yet the bubbles exist. This suggests that an outburst from a galaxy's core can happen in any galaxy, even the quietest. For astronomers, this is an important clue: perhaps the mysterious "odd radio circles" seen in the sky are also such bubbles, only viewed from the side, appearing as rings.
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.