a black hole-driven quasar in a nearby galaxy, recently shut off like a snuffed candle.

A black hole is a region of space whose gravitational force is so strong that nothing can escape from it. A black hole is invisible because it even traps light. The fundamental descriptions of black holes are based on equations in the theory of general relativity developed by the German-born physicist Albert Einstein. The theory was published in 1916.

One of the universe’s brightest lights, a black hole-driven quasar in a nearby galaxy, recently shut off like a snuffed candle. New observations of a bizarre cloud of glowing gas, and a nearby galaxy that illuminates it, show that the galaxy’s central light went dim sometime in the last 70,000 years.

The finding could reveal how supermassive black holes
help galaxies grow and evolve.

“This is the best view we will ever have of the host galaxy of a quasar,” said astrophysicist Kevin Schawinski of Yale University, lead author of a paper published online Oct. 26 in Astrophysical Journal Letters and cofounder of the crowdsourced space science project Galaxy Zoo. “It’s the nearest one to us, and the quasar is dead.”

Quasars are ravenous, supermassive black holes that lurk at the centers of some galaxies and devour gas and dust from a surrounding disk.

As the gas falls into the black hole, a process astronomers call accretion, friction heats the gas until it glows white-hot.

“Accretion transforms the darkest objects in the universe into the brightest,” said black hole expert Chris Done of the University of Durham in the U.K., who was not involved in the new study. “It’s like the final scream before it disappears forever below the event horizon.” Astronomers have long suspected that energy from quasars can help galaxies grow, a notion supported by the fact that bigger galaxies tend to have bigger central black holes. But until now, evolving galaxies have been impossible to observe.

A new clue comes from Hanny’s Voorwerp, a weird cloud of glowing green gas found in Galaxy Zoo’s archival telescope data in 2007. The Voorwerp (Dutch for “object” ) was thought to be lit up by a nearby quasar zapping it with a floodlight-like jet of ionizing radiation. But there was no nearby quasar to be found. The closest object was a wimpy, dim galaxy called IC 2497 between 45,000 and 70,000 light-years away from the Voorwerp. Both objects are about 730 million light-years from Earth, “which cosmologically speaking is our backyard,” Schawinski said. There were two possible explanations for the missing quasar. Either there’s so much gas and dust between Earth and the galaxy that the floodlight looks like a flashlight to us, but not to the Voorwerp, or the quasar died sometime between when its light hit the Voorwerp and now.

To test the first idea, Schawinski and colleagues observed the galaxy with two space telescopes, the Suzaku X-ray Telescope and the XMM-Newton X-ray observatory. These telescopes detect high-energy X-rays that would penetrate even the thickest clouds of dust. The team did see a few X-rays, but it was “a really weak, puny little source,” Schawinski said. “This source is way way way too weak to light up the Voorwerp,” he said. “It’s like trying to light up your house with an LED. It’s just not enough.”

As it appears today, the quasar is 100 to 10,000 times dimmer than whatever lit up the Voorwerp, Schawinski and colleagues concluded. The quasar must have shut down sometime in the last 45,000 to 70,000 years, while its light was still traveling to the Voorwerp, and left the cloud as a ghostly echo of the dead quasar’s former brilliance. Because the quasar switched off so quickly, the accretion disk surrounding the black hole must have been relatively small, Done says.

That’s consistent with some models of how black holes gobble up their surroundings. “Those models are quite hard to test,” Done said. “This might tell us is that our ideas about accretion do actually work. I’m quite excited about it.”

The quasar corpse also provides a unique laboratory for studying how galaxies evolve shortly after their central engines quiet down. “It’s certainly giving us a view of a very particular phase of the evolution of quasars, and one that we can’t normally study,” Done said. The discovery would never have happened if not for Galaxy Zoo, Schawinski notes. “We would have never found the nearest quasar to us, this system that will ultimately teach us so much about black holes and galaxies and how they evolve together, if it wasn’t for citizen science and the internet,” he said. “It’s so much cooler than anything we could have imagined.”

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