At least 240 million years ago a supermassive black hole underwent an enormous explosion, shooting out a jet that punched a tremendous cavity in the plasma surrounding its galaxy. The event was so large, comfortably exceeding anything we know of since the Big Bang, that when astronomers first detected its legacy they assumed they must be misinterpreting what they had found.

The possibility of such an event first came to light in 2016 when X-ray telescopes detected a gaping cavity in the intergalactic plasma in the Ophiuchus cluster of galaxies, the sky's second brightest X-ray cluster. The possibility of this having been caused by a black hole explosion was raised at the time, but considered unlikely precisely because it would be five times larger than anything comparable. Now an international team have announced in The Astrophysical Journal that examination of the same region with the Murchison Widefield Array (MWA) and the Giant Metrewave Radio Telescope indicates that is exactly what happened.

“We’ve seen outbursts in the centers of galaxies before but this one is really, really massive,” Professor Melanie Johnston-Hollitt, from the Curtin University node of the International Center for Radio Astronomy Research, said in a statement. (Before continuing, we would like to draw your attention to the statement's URL, www.icrar.org/kaboom)

Co-author Dr Simona Giacintucci of the US Naval Research Laboratory added: “You could fit 15 Milky Way galaxies in a row into the crater this eruption punched into the cluster’s hot gas.”

Johnston-Hollitt told IFLScience: “We know these explosions are the result of material falling into the black hole from their accretion disk.” However, she added, astronomers still don't understand why an ongoing process of matter only sometimes causes explosions, and why some are so much larger than others.

Equally mystifying is why this particular black hole, which is not especially large or apparently unusual in other ways, should be the one to smash the previous record.

Despite currently standing out, this event was probably not unique. Johnston-Hollitt explained to IFLScience these explosions leave behind synchrotron radiation caused by electrons spiraling in a magnetic field. These initially produce a wide spectrum of radio waves, but as time goes on, “emissions drop off at high frequencies first.”

Until recently, radio astronomy has concentrated on frequencies too high to detect an event like this unless it was very recent. It's only as the MWA has pushed into lower frequencies that this detection became possible.

The MWA currently has 2,048 antennas, but this will soon increase to 4,096, and Johnston-Hollitt told IFLScience the doubling in size will lead to something like a 10-fold increase in sensitivity as astronomers become able to distinguish signals from background noise. This will allow us to see similar phenomena much more distant than the Ophiuchus cluster, which is about 390 million light-years away.

Non-astronomers might imagine such an event being catastrophic for the galaxy in which it occurred, and particularly any inhabitants it might have had. However, Johnston-Hollitt explained the galaxy is fine. For one thing, the explosion lasted hundreds of millions of years, rather than being something sudden. Moreover, the jets are quite narrow as they leave the black hole and perpendicular to the accretion disk – which lies in the plane of the galaxy. Consequently, while a few unfortunate stars may have been in the wrong place, the vast majority were safely out of harm's way.

Such jets are normally emitted in opposite directions, but curiously, no equivalent radiation has been detected in the opposite direction. The paper notes that if plasma densities were lower there the surviving radio frequencies would be beyond our current capacity to detect.