In our many accolades of citizen science, nothing stands taller among discoveries than the strange object Hanny van Arkel found in archived images of the night sky in 2008.

After catching sight of it, courtesy of Hanny and Galaxy Zoo, astronomers were determined to learn more about Hanny's Voorwerp (Hanny's "object" in Dutch).    Now they say they have discovered that  Hanny's Voorwerp represents a 'snapshot in time' that reveals surprising clues about the life cycle of black holes.

In a new study, astronomers have confirmed that Hanny's Voorwerp is a large cloud of glowing gas illuminated by the light from a quasar—an extremely energetic galaxy with a supermassive black hole at its center. The twist described in Astrophysical Journal Letters is that the quasar lighting up the gas has since burned out almost entirely, even though the light it emitted in the past continues to travel through space, illuminating the gas cloud and producing a sort of "light echo" of the dead quasar.

Hanny's Voorwerp
Hanny's Voorwerp and IC 2497 from 2008. Credit: Dan Smith, Peter Herbert, Matt Jarvis&the ING.

"This system really is like the Rosetta Stone of quasars," said Yale astronomer Kevin Schawinski, a co-founder of Galaxy Zoo and lead author of the study. "The amazing thing is that if it wasn't for the Voorwerp being illuminated nearby, the galaxy never would have piqued anyone's interest."

The team calculated that the light from the dead quasar, which is the nearest known galaxy to have hosted a quasar, took up to 70,000 years to travel through space and illuminate the Voorwerp, meaning the quasar must have shut down sometime within the past 70,000 years.

Until now, it was assumed that supermassive black holes took millions of years to die down after reaching their peak energy output. However, the Voorwerp suggests that the supermassive black holes that fuel quasars shut down much more quickly than previously thought. "This has huge implications for our understanding of how galaxies and black holes co-evolve," Schawinski said.

"The time scale on which quasars shut down their prodigious energy output is almost entirely unknown," said Meg Urry, director of the Yale Center for Astronomy&Astrophysics and a co-author of the paper. "That's why the Voorwerp is such an intriguing—and potentially critical—case study for understanding the end of black hole growth in quasars."

Although the galaxy no longer shines brightly in X-ray light as a quasar, it is still radiating at radio wavelengths. Whether this radio jet played a role in shutting down the central black hole is just one of several possibilities Schawinski and the team will investigate next.

"We've solved the mystery of the Voorwerp," he said. "But this discovery has raised a whole bunch of new questions."

Citation: Kevin Schawinski , Daniel A. Evans , Shanil Virani , C. Megan Urry , William C. Keel , Priyamvada Natarajan , Chris J. Lintott , Anna Manning , Paolo Coppi , Sugata Kaviraj , Steven P. Bamford , Gyula I. G. Józsa , Michael Garrett , Hanny van Arkel , Pamela Gay and Lucy Fortson, 'The Sudden Death of the Nearest Quasar', 2010 ApJ 724 L30 doi: 10.1088/2041-8205/724/1/L30