A joint Fermilab/SLAC publication

Hanny's voorwerp explained

11/26/08
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 Herbert, Isaac Newton Telescope.

Westerbork Synthesis Radio Telescope 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 Herbert, Isaac Newton Telescope.

Hanny's voorwerp, a mysterious giant green astronomical object found over a year ago now has a partial explanation, according to a press release from Astron, the Netherlands Institute for Radio Astronomy. It seems that a jet of energetic particles from a nearby black hole has cleared a path in the interstellar medium so that visible and ultraviolet light associated with the black hole can heat the cloud, ionizing the particles, and causing it to glow green.

Hanny var Arkel is a Dutch schoolteacher who found the object while participating in the Galazy Zoo project, which asks members of the public to look at numerous astronomical objects and classify them into different types of galaxies.

The object now known as Hanny's voorwerp (voorwerp means object in Dutch), is labelled by astronomers SDSS J094103.80+344334.2, a number referencing the Sloan Digital Sky Survey data set. The black hole resides at the center of the galaxy IC 2497, which is about 60,000 light years away from Hanny's voorwerp.

In the press release, Dr. Tom Oosterloo says he thinks he has seen such a phenomenon 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".

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