06 Aug 08

The strange green blob, dubbed 'Hanny's Voorwerp' is believed to be a light echo from a now dim quasar.

A Dutch schoolteacher has discovered a mysterious and unique astronomical object through Galaxy Zoo, a project involving a team of Oxford University scientists, which enables members of the public to take part in astronomy research online.

Hanny van Arkel, a primary schoolteacher from the Netherlands, came across the image of a strange gaseous object with a hole in the centre that has been described as a ‘cosmic ghost’ whilst using the www.galaxyzoo.org website to classify images of galaxies.

She posted about the image – which quickly became known as Hanny’s ‘Voorwerp’ after the Dutch for 'object’ – on the Galaxy Zoo forum and the astronomers who run the site began to investigate. They soon realised the potential significance of what they think is a new class of astronomical object and will now use the Hubble Space Telescope to get a closer look at ‘Hanny’s Voorwerp’.

‘At first we thought it was a distant galaxy,’ said Dr Chris Lintott of Oxford’s Department of Physics, a galaxyzoo.org team member, ‘but we realised there were no stars in it so that it must be a cloud of gas.’ What was particularly puzzling to astronomers was that the gas was so hot – more than 10,000 degrees Celsius – when there were no stars in the vicinity to heat it up.

‘We now think that what we’re looking at is light from a quasar – the bright, stormy centre of a distant galaxy powered by a supermassive black hole,’ said Dr Lintott. ‘The quasar itself is no longer visible to us but its light continues to travel through space and the Voorwerp is a massive ‘light echo’ produced as this light strikes the gas.’

The black hole at the centre of the galaxy, IC 2497, is now ‘turned off’ – which is why the quasar has gone dim – but around 100,000 years ago the quasar was bright enough to have been visible from the Earth through a small, inexpensive telescope.

Dr Lintott added: ‘From the point of view of the Voorwerp the galaxy looks as bright as it would have done before the black hole turned off – it’s this light echo that has been ‘frozen in time’ for us to observe. 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.’

Smaller light echoes have been noted around supernovae before but never anything of the scale and shape of the Voorwerp. As yet nobody has a sensible explanation for the hole that runs through its centre.

‘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,’ said Hanny van Arkel. ‘It was a fantastic present to find out on my 25th birthday that we will get observational time on the Hubble Space Telescope to follow-up this discovery.’

During the last year 50 million classifications of galaxies have been submitted on one million objects at www.galaxyzoo.org by more than 150,000 armchair astronomers from all over the world.

The next stage of Galaxy Zoo will ask volunteers for more detailed classifications, making it easier to identify more unusual objects such as Hanny's Voorwerp.

Above: Hanny van Arkel, who made the surprising cosmic discovery.

• The Galaxy Zoo team includes scientists from the University of Oxford, the University of Portsmouth and Johns Hopkins University (USA), and Fingerprint Digital Media of Belfast. Key contributors to this stage of the project were Bill Keel from the University of Alabama (USA), Dan Smith (Liverpool John Moores University) Peter Herbert and Matt Jarvis (University of Hertfordshire) and Nicola Bennert (University of California Riverside, USA).
• Details of the discovery are included in a paper submitted by the team to the Monthly Notices of the Royal Astronomical Society.
• The new digital images used in Galaxy Zoo were taken using the robotic Sloan Digital Sky Survey telescope in New Mexico.