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Westerbork Telescope reveals nature of Hanny’s Voorwerp

New observations made by radio telescopes have finally revealed the nature of the bizarre object known as “Hanny’s voorwerp” (SDSSJ094103.80+344334.2).

Published by the editorial team, 30 December 2008

The Voorwerp was discovered by Hanny van Arkel, a Dutch school teacher and an enthusiastic volunteer of the Galaxy Zooproject. While surfing through hundred’s of images, Hanny noticed a huge green irregular cloud of gas of galactic scale, located about 60,000 light years from a nearby galaxy, IC2497. 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 (> 15000 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 IC2497 and the Voorwerp with the Westerbork Synthesis Radio Telescope (WSRT) and an e-VLBI array in which the WSRT also participated.

Caption to the image above: WSRT observations reveal a radio 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 reveal a huge reservoir of hydrogen gas (coloured orange) that probably arose from a previous encounter between IC2497 and another galaxy. The presence of strong neutral hydrogen absorption (top right plot) argues that the central regions of IC2497 are highly obscured. Credit: Main image left and top right hand corner (ASTRON). Hanny’s voorwerp (bottom right) Dan Smith, Peter Herbert, Matt Jarvis & the ING.

The picture that is emerging from these data is that a jet of highly energetic particles is being generated by a massive black hole at the centre of IC2497. “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. “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 remaining question 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 extended across hundreds of thousands of light years”, says Dr. Gyula Józsa, another member of the team. According to Józsa the total mass of gas is about 5000 million times the mass of the 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 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 IC2497 – all the evidence points towards a group of galaxies at the tip of the stream being responsible for this freak cosmic accident”. Hanny van Arkel, who is visiting the team at ASTRON this week 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 observtions with the WSRT and with other telescopes soon.



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