SEATTLE, WASHINGTON—In 2007, Dutch secondary school biology teacher Hanny van Arkel spotted something mysterious in the night sky. Combing through Galaxy Zoo, an online database set up to enlist the public's help in classifying galaxies, she came across a glowing green smudge of light approximately 650 million light-years away. The object, which became known as Hanny's Voorwerp (Dutch for "object"), is one of the most mysterious in the universe. Now, detailed Hubble Space Telescope images and new x-ray observations presented here today at the 217th meeting of the American Astronomical Society may finally confirm what it is.
Initial imaging of Hanny's Voorwerp by a wide range of telescopes on the ground and in space indicated that it was a giant cloud of hot gas. Astronomers speculated that the glow was caused by irradiation from a neighboring galaxy known as IC 2497. The idea is that the galaxy harbors a giant black hole in its core that once gobbled up gas and stars, emitting two opposing jets of hot gas and high-energy radiation. Such active galaxies are also known as quasars. When the radiation from this object hit the gas cloud, it excited oxygen atoms, causing the cloud to glow green.
The new x-ray observations, presented by Yale University astronomer Kevin Schawinski, reveal that the quasar is no longer active, probably because the black hole ran out of food. But scientists believe it shut down quite recently, because the Voorwerp is still glowing. Given that light from IC 2497 takes tens of thousands of years to reach the Voorwerp, astronomers speculate that the quasar must have turned off less than 200,000 years ago. That means it shut down much faster than scientists thought possible.
The new Hubble observations, obtained in April 2010, also confirm the dead-quasar hypothesis. In particular, they reveal clusters of young stars in the Voorwerp, some of which are no older than a million years; they're the yellowish areas in the upper right part of the object (see picture). According to astronomer and team leader William Keel of the University of Alabama, Tuscaloosa, the presence of these young stars indicates that jets of fast-moving particles—which are ejected by quasars—bombarded the gas cloud. As a result, the gas got compressed and new stars were born.
Other Hubble measurements revealed an expanding bubble of matter in the galaxy's core. "Both the star clusters in Hanny's Voorwerp and the expanding bubble in the galaxy's core hint at a strong outflow of gas, fostered by a quasar," says Keel.
The new Hubble images are the most detailed observations of a quasar host galaxy ever, says Schawinski. Most quasars are much farther away, and usually the host galaxies can't be studied well because the quasar light is so overwhelmingly bright. Moreover, he says, the observed rapid shutdown of the quasar in IC 2497 will help astronomers better understand the feasting physics of supermassive black holes.