Dead Quasar Leaves Ghost Behind

/ Source: Discovery Channel

Quasars are perhaps the most ravenous and immortal beasts in the universe. They are bloated black holes containing the mass of millions of suns and are usually spotted making quite a ruckus as they gnash up and swallow screaming matter which falls within their gravitational grip.

That's why astronomers are surprised to announce that have caught a delayed glimpse of the death of one such greedy colossus, reflected in a sort of intergalactic mirror.

The mirror, in this case, is a vast blob of gas about 70,000 light-years away from the center of a spiral galaxy, where the quasar resides. The gas cloud has been called Hanny's Voorwerp (Dutch for Hanny's Object), after the citizen scientist involved in the discovery of the previously inexplicable object in archival photographs of galaxies.

"This blob definitely was there in data that had been taken years and years before," said Yale University astronomer Kevin Schawinski, a co-founder of Galaxy Zoo citizen science project and lead author of the study accepted for publication in Astrophysical Journal Letters. "But nobody was looking for it." And it definitely was not immediately clear what it was.

Now astronomers have determined that the kelly green Voorwerp seen in the image is an optical echo of the quasar when it was shining brightly, despite the fact that the quasar is currently almost undetectable.

This is possible only if the quasar was feasting -- and therefore shining with the high-energy emissions of dying matter about to fall into the black hole -- at some time within the 70,000 years it takes the light from the once-bright quasar to cross space and reach the Voorwerp.

In other words, the Voorwerp is an optical echo of the now dead quasar.

To confirm that a dead quasar is at the center of the galaxy near the Voorwerp, Schawinski and his colleagues looked for the quasar in X-rays, which is the kinds of light emitted by matter being torn apart as it enters a black hole.

"We did find something, but the luminosity is 1/10,000th of what would be needed" to light up the Voorwerp, Schawinski told Discovery News.

What makes the discovery scientifically important is that it's the first evidence of how fast a quasar can shut down, Schawinski explained. Quasars had been thought to perhaps fade away as the material swirling around the black holes in "accretion disks" gets eaten up. But the Voorwerp suggests they can shut down very suddenly.

"It tells us the size of the accretion disk would have to be relatively small," commented astronomer Chris Done of the University of Durham,in Durham, U.K. This fits with one approach quasar modelers have tried to simulate, she said, but does not mean that's the only way quasars shut off. To determine that more Voorwerps need to be found and studied. That will take a lot more searching.

"You have to go through hundreds of thousands of (photographs of) objects to find them," said Done.

Since Voorwerps are not the sort of thing a computer can be easily programmed to identify, the search for more requires human eyes like those of the Galaxy Zoo, said Done.

That's exactly the plan for the Galaxy Zoo citizen scientists, said Schawinski.