Radio observations reveal a 'jet' (white contours) emanating from the centre of the nearby galaxy IC 2497, headed straight in the direction of Hanny's Voorwerp (green). The observations also show a huge reservoir of hydrogen gas (coloured orange) that probably arose from a close encounter between IC 2497 and another galaxy.
Scientists identify Galaxy Zoo mystery object
27 Nov 2008
New observations made by radio telescopes have finally revealed the nature of the bizarre object known as 'Hanny's Voorwerp'. The Voorwerp ( Dutch for 'object') was discovered by Hanny van Arkel, a Dutch school teacher and an enthusiastic volunteer of the Galaxy Zoo project.
While surfing through hundreds of images, Hanny noticed a huge, green irregular cloud of gas of galactic scale, located about 60,000 light-years from the galaxy IC 2497.
The object has had astronomers scratching their heads for over a year now—the extent of the cloud is enormous and the gas is extremely hot (more than 15,000 degrees Celsius) but paradoxically it is devoid of stars.
An international team of astronomers, led by Prof. Mike Garrett (ASTRON/Leiden) , and including Hanny van Arkel herself, have observed IC 2497 and the Voorwerp with the Westerbork Synthesis Radio Telescope (WSRT) and a radio telescope array in which the WSRT also participated.
The picture that is emerging is that a 'jet' or outburst of highly energetic particles is being generated by a massive black hole at the centre of IC 2497. 'It looks as though the jet emanating from the black hole clears a path through the dense interstellar medium of IC 2497 towards Hanny's Voorwerp', says Garrett.
Hanny's Voorwerp, a green galactic blob discovered by a Galaxy Zoo volunteer, Hanny van Arkel. >
This cleared channel permits the beam of intense optical and ultraviolet emission associated with the black hole, to illuminate a small part of a large gas cloud that partially surrounds the galaxy. The optical and ultraviolet emission heats and ionises the gas cloud, thus creating the phenomena known as Hanny's Voorwerp.
One question remaining is, where does all the hydrogen gas come from?
'There is a lot of gas out there—the WSRT observations detect a huge stream of gas that is extending across hundreds of thousands of light-years', says Dr Gyula Jozsa, another member of the team. According to Jozsa the total mass of gas is about 5,000 million times the mass of our Sun.
It's something Dr Tom Oosterloo thinks he has seen before: 'It has all the hallmarks of an interacting system—the gas probably arises from a 'tidal' (or gravity) interaction between IC 2497 and another galaxy, several hundred million years ago'.
Oosterloo also thinks he can identify the culprits. 'The stream of gas ends three hundred thousand light-years westwards of IC 2497—all the evidence points towards a group of galaxies at the tip of the stream being responsible for this freak cosmic accident'.
< School teacher Hanny van Arkel, who discovered the Voorwerp
Hanny van Arkel is impressed: 'I'm happy we are making progress. Apparently the more we learn about the Voorwerp, the more intriguing it becomes'.
Garrett and his team agree: 'We think the Voorwerp has a few more secrets to reveal'.
The team plan much deeper observations with the WSRT, and with other, higher-resolution radio telescopes soon.
ASTRON is part of the Netherlands Organisation for Scientific Research (NWO).
More information: Galaxy Zoo
Adapted from information issued by ASTRON / Dan Herbert, Isaac Newton Telescope.
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