Cosmic ghost haunts

Galaxy Zoo
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: August 5, 2008

A volunteer in the Galaxy Zoo campaign has identified the most mysterious and unique object found yet amongst the menagerie of one million galaxies available for online classification by armchair astronomers across the globe: a cosmic ghost.

“This discovery really shows how citizen science has come of age in the Internet world,” says Professor Bill Keel of the University of Alabama, a galaxyzoo.org team member.

Just like any other volunteer Galaxy Zoo astronomer, Hanny Van Arkel is an ordinary member of the public – a dutch school teacher – but has found a distinctly extraordinary object that some observers are calling a cosmic ghost, which sports a huge central hole over 16,000 light years across.

Henny's Voorwerp is the green blob of gas in the centre of this image, sporting a 16,000 light year diameter hole and believed to be a light echo from the bright, stormy centre of a distant galaxy that has since turned out its lights. Image: Dan Smith, Peter Herbert, Matt Jarvis & the ING.

The object has also been nicknamed ‘Hanny’s Voorwerp’ (Voorwerp is dutch for ‘object’) and has been awarded time on the Hubble Space Telescope for follow-up work at a later date. But for now, the Galaxy Zoo-keepers can only speculate as to the origins and nature of the eerie cosmic ghost that haunts the image of a relatively normal frame of galaxies.

"What we saw was really a mystery,” says Schawinski, co-founder of the Galaxy Zoo, “It could have been in our Solar System, or at the edge of the Universe." Even after telescopes around the world were trained on the mysterious object, the astronomers were none the wiser because the Voorwerp didn't appear to contain any stars. Instead, it was made entirely of 10,000 degree Celsius hot gas that could only be illuminated by something powerful lurking in the vicinity, and so the team turned to nearby galaxy IC 2497 for clues.

“We think that in the recent past the galaxy IC 2497 hosted an enormously bright quasar," says Schawinski. "Because of the vast scale of the galaxy and the Voorwerp, light from the past still lights up the nearby Voorwerp even though the quasar shut down sometime in the last 100,000 years, and the galaxy's black hole itself has gone quiet." Similar light echoes have been seen around supernovae that exploded decades or centuries ago, but Hanny’s Voorwerp could be the nearest example of such a luminous quasar, with the next nearest active quasar – 3C 273 – a massive 1.7 billion light years further away.

"It's amazing to think that this object has been sitting in the
archives for decades and that amateur volunteers can help by spotting things like this online," says Hanny van Arkel, discoverer of the ‘ghost’.

The Galaxy Zoo project was conceived by Schawinski and his colleague Chris Lintott at Oxford University as a means to classify millions of galaxies in a way that an automated classification system would not be able to achieve. The project has so far been an overwhelming success and during the last year, over 150,000 armchair astronomers from all over the world submitted around 50 million online classifications for a set of one million galaxies. The next stage of the Galaxy Zoo enterprise will focus on more unusual astronomical objects, and will perhaps even uncover more beautiful mysteries like Hanny’s Voorwerp.

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