Shining New Light on “Hanny’s Voorwerp”

Amateur astronomers do what they do because they love exploring the universe by whatever means possible. They probe the sky with telescopes, binoculars, or just their eyes alone. It's not about becoming famous for making some some remarkable discovery. They're simply curious.

Hanny van Arkel
Now famous for the voorwerp (object) she chanced upon in 2007, Hanny van Arkel teaches biology at Citaverde College in Heerlen, the Netherlands.
Edd Edmondson
But once in a while all that looking turns up something amazing. That's what happened on August 13, 2007, to a 24-year-old Dutch schoolteacher named Hanny van Arkel. She'd been participating in a citizen-science project called Galaxy Zoo, which had enlisted volunteers to characterize roughly a million galaxies in images acquired robotically by the Sloan Digital Sky Survey.

Galaxy Zoo had only running for a few weeks when van Arkel spotted a mysterious blue blob near the spiral galaxy IC 2497 in Leo Minor. She had no idea what it was, nor did any of the project's "Zookeepers." (Here is her original post about it.) It looked like a glowing nebula, but it was huge — nearly 100,000 light-years across.

Since both "Hanny's Voorwerp" (Dutch for Hanny's Object) and IC 2497 are about 650 million light-years away, cosmologists speculated that the colorful cloud was fluorescing due to radiation beamed from a supermassive black hole at the galaxy's core.

Hanny's Voorwerp and IC 2497
Discovered in 2007, the bizarre object dubbed Hanny's Voorwerp is the only visible part of a 300,000-light-year-long streamer of gas stretching around the spiral galaxy IC 2497. Click here for larger views.
NASA / ESA / W. Keel / Galaxy Zoo Team
But IC 2497 shows no sign of having an active galactic nucleus, or AGN. Also known as quasars, these churning powerhouses typically emit unimaginable amounts of light and radiation, far outshining their host galaxies. So a healthy debate ensued among observers and thorists. Did IC 2497 harbor a stealth quasar that somehow switched itself off recently?

Finally, astronomers have some answers, thanks in part to a dramatic view of Hanny's Voorwerp taken last year by the Hubble Space Telescope and released last week at a meeting of the American Astronomical Society. Hanny's Voorwerp appears to be basking in the afterglow of an AGN that shut itself down within the past 200,000 years — a moment ago, astronomically speaking. According to a team led by Kevin Schawinski (Yale University), the shutdown likely was much more recent, within the past 70,000 years.

AGNs can sometimes flicker in brightness, but nothing this dramatic has ever been seen. "We just missed catching the quasar," notes William Keel (University of Alabama), who presented the Hubble findings last week.

Hanny's Voorwerp explained
Astronomers suspect that Hanny's Voorwerp is part of a long tail of gas created when another galaxy merged with IC 2497. Click on the image for a larger view.
NASA / ESA / A. Feild (STScI)
Radio observations show that Hanny's Voorwerp is actually part of a long stream of gas about 300,000 light-years long connected to IC 2497. This is apparently a tidal tail strung out when another galaxy nearly collided with the big spiral roughly a billion years ago. (More evidence for this close encounter comes from the distorted shape of IC 2497's arms.)

But only Hanny's Voorwerp is in the galactic spotlight, so to speak. The distinct circular hole in its center is most likely the shadow of an object near the galaxy's center that is blocking part of the beam. Hubble's view reveals that clusters of stars are forming along the galaxy-facing edge of the cloud.

You can learn more about how Keel and his team have interpreted the Hubble observations here. But don't miss the chance to learn more about Hanny van Arkel herself. She's got a terrific website, and her now-famous discovery has even been immortalized in an online comic book.