A team of astronomers from India has discovered an extremely rare galaxy located 9 billion light-years away towards the constellation Cetus. 

The object, called J021659-044920, was observed in radio waves using the Giant Metrewave Radio Telescope by Prathamesh Tamhane, a student at the Indian Institute of Science Education and Research. He and his colleagues from the National Centre for Radio Astrophysics in Pune, India, saw that the object is a giant radio galaxy, and it has been emitting radio waves in two opposite lobes (see image above) that measure 4 million light-years from one end to the other – that's 40 times the diameter of the Milky Way.

Radio galaxies with lobes up to a million light-years tend to be quite common, but giant radio galaxies are few and far between, especially in the early universe. But that’s not all: The lobes have started to fade, which the team took as an implication that the galaxy has stopped being active. 

While the supermassive black hole and other structures in J021659-044920 are mostly undetectable using the radio telescope, follow-up observations using infrared and X-ray telescopes have confirmed the first assessment made by the team. The galaxy is a dying relic of the giant radio galaxy it once was. 

Once the supermassive black hole at the center of a galaxy starts accreting matter on itself, it produces powerful jets of charged particles and magnetic fields. The jets generate strong emissions of radio waves that can be significantly larger than the galaxy that emitted them. Radio lobes like the ones belonging to J021659-044920 probably took hundreds of millions of years to produce. 

It is rare but possible for black holes to stop emitting these jets. When that happens, the radio wave signature tends to fade away after a few million years as they are not being replenished. This is what happened to J021659-044920. 

Observing the lobes just as they begin to fade is a unique opportunity to better understand the dynamics and the life cycle of radio galaxies. 

The team published their discovery in the Monthly Notices of the Royal Astronomical Society.