What is Hanny's Voorwerp? Two years ago a Dutch schoolteacher named Hanny van Arkel, poring over sky-survey images, noticed a unique, faint blue object (voorwerp is Dutch for object) near a faint galaxy. Astronomers soon suspected that the starless blob is a cloud of intergalactic gas lit up by an active galactic nucleus (AGN) in the galaxy right next to it.
However, the galaxy has no active nucleus. At least not as we see it now. Two groups have come up with different accounts of why it's missing.
Hanny van Arkel did not discover the blob by chance. She was one of the many volunteers in the Galaxy Zoo project, which enrolls people to classify nearly a quarter million galaxies in pictures from the 2.5-meter Sloan Digital Sky Survey (SDSS) telescope in New Mexico. On one of the pictures assigned to van Arkel, in Leo, the odd object at right caught her eye. It looked blue on the extended-spectrum SDSS image, though it actually proved to be green.
“What’s the blue stuff below?” she asked on the Galaxy Zoo forum. Since then, astronomers have used the 4.2-meter William Herschel Telescope on La Palma in the Canary Islands to obtain better images of the object.
From the outset, astronomers assumed that Hanny's Voorwerp is fluorescing ionized gas rather like the (much smaller) emission nebulae in the Milky Way. If so, ultraviolet light — lots of it — must be coming from somewhere to ionize it. IC 2497 was the obvious suspect, since both blob and galaxy are about 700 million light-years away.
As it turns out, the blob is a small part of a gas cloud bigger than the galaxy itself, though with less mass. Only the part exposed to ionizing ultraviolet radiation glows — as happens when you aim a flashlight at part of a cloud. “That part of the cloud you will see; the rest, you will not see,” says Gyula Jozsa (Netherlands Institute for Radio Astronomy, ASTRON), first author of a recently published paper on the Voorwerp.
Jozsa and collaborators also saw something that wasn’t quite the beam of light aimed at the blob, but more like the hand that pushes the branches away to let the light through. They found lots of radio emission between the center of the galaxy and the blob, as would be produced by the jet of plasma one expects from an AGN. The jet would be removing dust within part of the galaxy. “The picture is that this jet has cleared a path for the light to reach that region of the gas cloud,” Jozsa says.
AGNs are very massive black holes that emit high-energy radiation and jets of gas as they collect and swallow surrounding matter. If the AGN is so bright that it outshines its host galaxy, it's called a quasar.
Even though nobody has seen IC 2497's active nucleus, there's an indirect sign of it. “We see very compact radio emission, which is very difficult to explain without an AGN,” says co-author Mike Garrett. On the other hand, the expected X-ray signature is missing; it couldn’t reasonably have escaped the Swift satellite.
Enter dust again: “You’re looking at a galaxy that is very dusty, especially toward the center,” says Garrett. “What we see is a lot of obscuring material in front of this AGN,” says Jozsa. They blame the dust in our line of sight for hiding the UV and X-rays from the galaxy's nucleus as well as the visible light.
Others beg to differ. “We say that cannot be an active quasar,” says Chris Lintott (Oxford), one of the astronomers running the Galaxy Zoo project, who also call themselves Zookeepers. Lintott and other Zookeepers contributed to Jazso and Garrett’s paper, but they are putting the finishing touches to an article of their own.
“I think we can show that if there was extra dust, it would be heated,” Lintott says. As a consequence, he says, “the dust that you’d need to hide it would be much brighter in the infrared than what we see.”
Lintott believes that instead, the quasar may have switched off — or temporarily decreased in intensity. Remember that the green blob is many thousands of light-years from the galaxy. This causes a time delay for radiation traveling from the center of IC 2497 to the blob to us, compared to a direct line from IC 2497 to us. Lintott reckons the quasar would have had tens of thousands of years to die down while the "light echo" from the green blob is still shining.
Explaining that the quasar-switch-off theory is not far-fetched, Lintott says: “We see some of them flickering on short timescales.” Moreover, there's evidence that the Milky Way's own central black hole flared up a mere 60 years ago, judging by suspected X-ray light echoes around it. The Chandra X-Ray Observatory also recently found a much larger cloud of X-ray-emitting gas continuing to glow near a very distant galaxy long after the AGN that energized the gas turned off.
If it's confirmed that a full-up quasar switched off in IC 2497, that would be big news: “We’ve never caught one in the act,” Lintott says. But Garrett cautions, “Because you don’t see it doesn’t mean it’s not there.”
Either way, we’re in for some interesting physics. Says Lintott, “If it’s hidden, it must be strange indeed.”