Ghost Story

August 08, 2008

Do you believe in ghosts?

A Dutch school teacher who volunteered to help catalog galaxies might be inclined to be less skeptical, after she found a weird, green-tinged blob of gas with a hole at the center in an image collected by the Sloan Digital Sky Survey telescope in New Mexico. Hanny van Arkel, 25, created quite a bit of buzz in astronomical circles when she posted about her finding online at Galaxy Zoo, a UK-based project that lets actual members of the public engage in astronomy research.

Dubbed "Hanny's Voorwerp" ("object" in Dutch, although I like to think of it as "the Green Goblin"), astronomers concluded she'd found a new class of astronomical object -- one that didn't contain any stars. It's just a clump of extremely hot gas (about 10,000 degrees Celsius), with a mysterious source of illumination -- mysterious because there's no obvious sign of such a source, yet scientists knew it had to be there in order for the gas to be so darn hot.

This week,an astrophysicist at Yale University named Kevin Schawinski -- one of the founders of Galaxy Zoo, along with my co-blogger Chris Lintott and several others -- announced that it is likely the source of illumination is a nearby galaxy, IC2497.

The theory goes that IC2497 once housed a very bright quasar, rare celestial objects powered by supermassive black holes. What we're seeing is a kind of "light echo" that has been frozen in time long enough that van Arkel, and other astronomers, were able to observe it.

How can light be "frozen" in time? Well, remember that the light we see on earth has traveled a long, long way before it reaches us, so when we look through a telescope, we're sort of traveling back in time -- observing events that took place long ago. The same is true for the Voorwerp.

That hole at the center of Hanny's Voorwerp is enormous, spanning some 16,000 light years across. So even though the quasar died out some 100,000 years ago, along with the black hole at the center of the galaxy, from the Voorwerp's perspective, the galaxy seems as bright as it when the quasar was blazing full-force. Supernovae that exploded centuries ago sometimes leave similar light echoes. I loved Chris's quote about this in the official press release: "It's rather like examining the scene of a crime where, although we can't see them, we know the culprit must be lurking somewhere nearby in the shadows."

Galaxy Zoo isn't the first attempt to engage the public in scientific research by amassing an army of citizen scientists. SETI@Home paved the way in 1999, setting up a distributed computing grid of home computers connected by the Internet to search for evidence of possible radio transmissions from extraterrestrial intelligence. All you need to do is download the free software program from the SETI@Home Website, and you're ready to start analyzing chunks of data collected by the Arecibo Radio Telescope. At last count, there are over 5.2 million people worldwide participating in the project. Sadly, to date, there's been no sign of a signal from ET, although astronomers have identified targeted "areas of interest."

In 2005, as part of the World Year of Physics, the American Physical Society launched Einstein@Home, a similar distributed computing project aimed at searching for gravitational waves (ripples in the fabric of space-time). Einstein@Home also enlists the processing power of home computers, in this case, to analyze the massive amounts of data collected by the Laser Interferometer Gravitational Observatory (LIGO). It's still running, too, although it hasn't found anything yet either.

But Galaxy Zoo is special in that it rather quaintly relies on the individual human eye, which can often pick up unusual patterns that a computer program might miss. In an age of automation and massive scientific collaborations, it's nice to think that a young Dutch woman poring over pictures of the night sky could find an object that had been sitting in the archives for decades, waiting to be discovered.

Photo: Hanny's Voorwerp (a.k.a., "the Green Goblin"). Source: Sloan Digital Sky Survey/Galaxy Zoo, via Bill Keel/University of Alabama, another team member.