The city has long been asleep: not a single lit window, not a single new house. Only in the very center, in one lone building, someone forgot to turn off a spotlight. That's what a galaxy looks like when stars are no longer born, yet its central black hole still shines. Using the James Webb Space Telescope, scientists caught this 'spotlight' — a neon emission line that cannot be explained by anything other than the activity of a supermassive black hole.
This is the glow of neon atoms stripped of four electrons (a state known as [NeV]). To 'peel' an atom like that requires the energy of nearly a hydrogen bomb — about 97 electron-volts. Stars, even the hottest ones, can't do that. Only matter falling into a black hole, heated to millions of degrees, can switch on this beacon. In six galaxies where star formation had long ceased, the beacon was blazing.
Astronomers measured the width of the spectral lines — the spread of the light's jitter that reveals the frantic speed of gas near the black hole — and weighed these objects. Their masses range from 300 million to 3 billion solar masses. They keep growing, swallowing several solar masses of gas per year. That's comparable to, or even greater than, the rate at which new stars could still form in these galaxies.
After a galaxy stops forming stars, its central black hole throws itself a farewell feast on the remaining gas. This feast lasts for hundreds of millions of years. Without such episodes, computer models cannot explain how massive, quiescent galaxies appeared so early in the Universe. So the city with a lone spotlight still burning at its center falls asleep much more slowly than previously thought.
To produce the [NeV] neon line, temperatures of about a million degrees are needed — like in the Sun's core, but here the heat comes not from nuclear reactions but from gas rubbing against itself as it falls into the black hole.
In Arthur C. Clarke's novel 'Rendezvous with Rama,' a mysterious object wakes from a long sleep and turns on its engines — much like black holes in 'dormant' galaxies suddenly flare up again.