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Weird 'Voorwerp' in the spotlight

W. Keel / U. of Alabama, Tuscaloosa / NASA / ESA / Galaxy Zoo

In this image by Hubble Space Telescope, an unusual, ghostly green blob of gas appears to float near a normal-looking spiral galaxy. The bizarre object, dubbed Hanny's Voorwerp (Hanny's Object in Dutch), is the only visible part of a streamer of gas stretching 300 000 light-years around the galaxy, called IC 2497. The greenish Voorwerp is visible because a searchlight beam of light from the galaxy's core has illuminated it. This beam came from a quasar, a bright, energetic object that is powered by a black hole. The quasar may have turned off in the last 200 000 years.

The weird greenish flare-up as "Hanny's Voorwerp" represents the afterglow of a cosmic spotlight that switched itself to stealth mode no more than 200,000 years before, astronomers reported today.

New imagery from the Hubble Space Telescope helps explain how the spotlight might work ... and it also throws a fresh spotlight on Hanny van Arkel, the Dutch high-school teacher who discovered the object in 2007 as part of the citizen-science project known as Galaxy Zoo. Hanny and her "Voorwerp" (the Dutch word for "object") have already been the subject of multiple research papers and even a graphic novel.

Van Arkel was a special guest this week at the American Astronomical Society's winter meeting in Seattle, where the Hubble image was unveiled today. She told me that her students "think it's cool" that she has become a star in citizen-science circles, but that her discovery hasn't changed her life.

"I'm just Hanny," the 27-year-old said. "I'm just a teacher. I live a normal life." 

Hanny van Arkel says she's enjoying her visit to Seattle for the American Astronomical Society meeting. "Everybody is so nice," she says.

Van Arkel spotted the object as she was clicking through images for the Galaxy Zoo project, which has enlisted thousands of Internet users to classifying galaxies according to their shape. At first, she didn't know what to make of the Voorwerp, and neither did astronomers.

Hanny's Voorwerp turned out to be a small part of a 300,000-light-year-long streamer of gas, located about 650 million light-years from Earth. Scientists suggested that a quasar in a nearby galaxy, known as IC 2497, was shining on Hanny's Voorwerp, lighting up the oxygen in the streamer with a greenish glow. The only problem was that no quasar could be seen.

Eventually, astronomers spotted a radio source in the galaxy that was sending out weak emissions. "That's like seeing a bank of fog lit up by a floodlight, but when you look to where the floodlight is, you see a laser pointer," Yale astronomer Kevin Schawinski said today.

One possible explanation is that the supermassive black hole switched itself off rapidly. But another possibility is that the swirling disk of material surrounding the black hole switched the way that it gave off energy. Instead of radiating energy as quasar jets of light, the disk started throwing off kinetic energy, in the form of a shock wave of gas moving into the surrounding space. The high-resolution Hubble image supports the latter explanation.

Data from Hubble's Wide Field Camera 3 and Advanced Camera for Surveys pointed to an small yellowish area where gas from the galaxy was squishing Hanny's Voorwerp, giving rise to clusters of new stars.

"The star clusters are localized, confined to an area that is over a few thousand light-years wide," William Keel, an astronomer at the University of Alabama in Tuscaloosa who led the Hubble study, said in an image advisory. "The region may have been churning out stars for several million years. They are so dim that they have previously been lost in the brilliant light of the surrounding gas."

It's not unprecedented for the disks around black holes to switch their energy state, Schawinski said. Galactic X-ray binaries, which pair a compact object such as a black hole or a neutron star with a more normal star, can alternate between blazing with jets of light and throwing off material. But Schawinski said such behavior hasn't been documented before with supermassive black holes.

"This is actually the first example of this behavior we've seen," he said.

Astronomers say there's a chance the quasar could switch on again if more material is dumped around the galaxy's black hole. "The evidence suggests that IC 2497 may have merged with another galaxy about a billion years ago," Keel said. "The Hubble images show in exquisite detail that the spiral arms are twisted, so the galaxy hasn't completely settled down."

More about citizen science:

Want to find your very own Voorwerp? You can sign up for a citizen-science project at Zooniverse. Take your pick among Galaxy Zoo Mergers, Galaxy Zoo Hubble, Galaxy Zoo Supernovae, Moon Zoo, The Milky Way Project, Planet Hunters, Solar Stormwatch and Old Weather.

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