In this image by NASA's Hubble Space Telescope, an unusual, ghostly green blob of gas appears to float near a normal-looking spiral galaxy. The bizarre object, dubbed Hanny's Voorwerp (Hanny's Object in Dutch), is the only visible part of a 300,000-light-year-long streamer of gas stretching around the galaxy, called IC 2497.
Credit: NASA, ESA, W. Keel (University of Alabama), and the Galaxy Zoo Team
NASA's Hubble Space Telescope is helping to demystify one of the strangest objects in space — a huge, glowing, green blob of gas floating near a spiral galaxy.
Hubble has uncovered delicate filaments of gas and a pocket of young star clusters in the giant object, which is known as Hanny's Voorwerp ("Hanny's Object" in Dutch). Hubble's new photos provide the sharpest view yet of Hanny's Voorwerp, researchers said, and reveal a star-birthing region in the object.
"The star clusters are localized, confined to an area that is over a few thousand light-years wide," study leader William Keel, of the University of Alabama in Tuscaloosa, said in a statement. "The region may have been churning out stars for several million years. They are so dim that they have previously been lost in the brilliant light of the surrounding gas."
Keel presented his team's results today (Jan. 10) at the winter meeting of the American Astronomical Society in Seattle.
Citizen science comes up big
The new Hubble discoveries are the latest finds in an ongoing probe of Hanny's Voorwerp, which is named for Hanny van Arkel. Van Arkel, a Dutch schoolteacher, discovered the ghostly structure in 2007 while participating in the online Galaxy Zoo project.
Galaxy Zoo enlists the public to help classify more than a million galaxies catalogued in the Sloan Digital Sky Survey. The project has expanded to include the Hubble Zoo, in which the public is asked to assess images of tens of thousands of galaxies from the Hubble Space Telescope.
In the new study, researchers report that Hubble's cameras have uncovered star birth in a region of Hanny's Voorwerp that faces the spiral galaxy IC 2497, which is about 650 million light-years from Earth.
Radio observations had previously shown an outflow of gas from the galaxy's core. The new Hubble images reveal that the galaxy's gas is interacting with a small region of Hanny's Voorwerp, which is collapsing and forming stars. The youngest stars are a couple of million years old, researchers said.
An active galactic core
Recent X-ray observations had revealed why Hanny's Voorwerp caught the eye of astronomers in the first place. IC 2497's active core produced a quasar — a powerful beacon of light powered by a black hole — researchers said.
The quasar shot a broad beam of light in the direction of Hanny's Voorwerp, illuminating the gas cloud. Its bright green color comes from glowing oxygen, according to researchers.
"We just missed catching the quasar, because it turned off no more than 200,000 years ago, so what we're seeing is the afterglow from the quasar," Keel said. "This implies that it might flicker on and off, which is typical of quasars, but we've never seen such a dramatic change happen so rapidly."
The quasar's outburst also may have cast a shadow on Hanny's Voorwerp, giving the illusion of a gaping hole about 20,000 light-years wide in the object, researchers said. Hubble images reveal sharp edges around the apparent opening, suggesting that an object close to the quasar may have blocked some of the light.
A rough life for IC 2497
Other studies have revealed that Hanny's Voorwerp is not an island in space. Rather, it's part of a twisting rope of gas about 300,000 light-years long that wraps around the galaxy, researchers said.
The only optically visible part of the rope is Hanny's Voorwerp, which stretches from 44,000 light-years to 136,000 light-years from the galaxy's core.
The quasar, the outflow of gas that instigated the star birth and the long rope of gas point to a rough life for IC 2497, researchers said.
"The evidence suggests that IC 2497 may have merged with another galaxy about a billion years ago," Keel said. "The Hubble images show in exquisite detail that the spiral arms are twisted, so the galaxy hasn't completely settled down."
A recently dimmed quasar
In Keel's scenario, the galactic merger expelled the long streamer of gas from IC 2497 and funneled gas and stars into the center, which fed the black hole. The engorged black hole then powered the quasar, which launched two cones of light. One light beam illuminated part of the gas cloud, which is now called Hanny's Voorwerp.
About a million years ago, the researchers think, shock waves produced glowing gas near the galaxy's core and blasted it outward. The outburst may have triggered star formation in Hanny's Voorwerp. Then, less than 200,000 years ago, the quasar dropped in brightness by 100 times or more, leaving an ordinary-looking core.
New images of the galaxy's dusty core from Hubble show an expanding bubble of gas blown out of one side of the core, perhaps evidence of the sputtering quasar's final gasps. The expanding ring of gas is still too small for ground-based telescopes to detect, according to researchers.
"This quasar may have been active for a few million years, which perhaps indicates that quasars blink on and off on timescales of millions of years, not the 100 million years that theory had suggested," Keel said. He added that the quasar could light up again if more material is dumped into the black hole.