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Explaining Hanny's Voorwerp.

Two years ago a Dutch schoolteacher named Hanny van Arkel was poring over Sloan Digital Sky Survey images on the internet, classifying galaxies as part of the Galaxy Zoo project. Just south of the spiral IC 2497 she noticed a faint, vivid blue-green thing like nothing else in the sky (see page 73). Astronomers soon named it Hanny's Voorwerp, from the Dutch for "object." It seemed to be a starless blob of fluorescing intergalactic gas, most likely lit by a quasar-like active galactic nucleus in IC 2497 (see image at right).

Except the galaxy has no active nucleus. At least not now. Two groups have come up with different ideas of why it's gone missing.

Both the blob and the galaxy are about 700 million light-years away in Leo. The colorful Voorwerp, it turned out, is a small part of a gas cloud bigger than the galaxy itself; only the area exposed to ultraviolet light glows.

IC 2497 itself does show radio emission that suggests an active galactic nucleus (AGN) around a black hole, but the expected X-ray signature is missing. Dense dust might block X-rays from the nucleus while letting radio waves out, and a radio-bright jet might clear a tunnel through the dust for ultraviolet light to illuminate part of the neighboring cloud. But in that scenario the dust in the galaxy should be heated, and it's not.


Another theory is that the galaxy used to contain a quasar, but it turned off. The blob is thousands of light-years away from the galaxy. This introduces a time delay for radiation traveling from the center of IC 2497 to the blob to us, compared to our straight-line view of IC 2497. So the quasar has had tens of thousands of years to die down while its "light echo" from the blob remains shining.

This idea has precedent. The Milky Way's own central black hole, now very quiet, apparently flared up intensely a mere 60 years ago, judging by suspected X-ray echoes from material around it.

The image at left was taken with the 2.5-meter Isaac Newton Telescope in the Canary Islands. The Voorwerp's prominent ring with a hole is about 16,000 light-years wide.

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Title Annotation:News Notes; galaxy
Publication:Sky & Telescope
Geographic Code:4EUNE
Date:Sep 1, 2009
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