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Unravelling the mystery of Hanny’s Voorwerp

A group of researchers, led by Professor Michael Garrett, General Director of the Netherlands Institute for Radio Astronomy, ASTRON, have made new, high resolution radio observations of the region of space around Hanny’s Voorwerp, the mysterious, greenish gas cloud discovered by Dutch school teacher Hanny van Arkel.

Published by the editorial team, 24 June 2010

The astronomers undertook an observational campaign at radio wavelengths using the European Very Long Baseline Interferometry Network (EVN) and the UK’s Multi-Element Radio Linked Interferometer Network (MERLIN), pointing their telescopes at the centre of the neighbouring galaxy IC 2497. In these measurements, several radio telescopes across Europe and the UK were linked together in real-time, in order to gain a detailed picture of the very central region of the galaxy. They observed a field a few arcseconds (the 3600th part of a degree) across, with a spatial resolution of about 70 milliarcseconds (0.07 arcseconds).

According to Garrett: “the observations show two bright and very compact sources with broadband spectra that argue strongly for the existence of an active galactic nucleus (AGN) at the centre of the galaxy, IC2497. One of the sources appears to be identified with the base of the super-massive blackhole at the centre of the AGN itself, while the other one is likely to be the result of an energetic jet expelled by the black-hole and now interacting with the dense gas that surrounds IC2497”. The radiation output from the AGN is believed to heat Hanny’s Voorwerp to a temperature above 10000 degrees.

But this is only part of the story. It also appears that surrounding the AGN, a lot of extended radio emission is also observed. The researchers argue that this is radio emission associated with a nuclear starburst. “We knew that IC 2497 is forming stars, but we were susprised to find that the star formation seems to be concentrated in a very small central region, only 3000 light years across”, Hayden Rampadarath (University of Machester) explains. Garrett: “It is fairly unusual to find both vigorous star formation and AGN radio activity in the same system and on similar scales. It seems that IC2497 swings both ways”.

The radio observations show that in this small region, IC 2497 is producing stars with a total mass of 70 suns every year. This star formation rate is pretty large, especially in the local Universe – it is about 6 times higher than in the nearby ‘Starburst-Galaxy’ poster-child, M82.” “We know of only a few hundred of these types of galaxy, Luminous Infrared Galaxies, LIRGs, in the local universe”, Gyula Józsa, support scientist at ASTRON, adds, “but they must have been much more frequent in the past”. At half the universe’s age, most stars have been formed in LIRGs. It is also typical that for this kind of galaxy large amounts of dust obscure optical and ultraviolet light towards the observer.

The observations support the group’s earlier hypothesis that a hidden AGN in the centre of IC2497 is ionising a distinct region of gas that surrounds the giant galaxy. That distinct region is what we know as Hanny’s Voorwerp. Such phenomenae must be rare in the local Universe, as they depend on a specific geometry of the observer, galaxy and gas, plus the interaction of several galaxies in the field in order to fuel the AGN and the starburst, and to create the gas reservoir that forms part of the Voorwerp.

For more information, please contact:


Gyula Józsa , Support scientist, ASTRON. Tel.: +31 521 595 100. E-mail:

Mike Garrett, General Director, ASTRON. Tel: +31 521 595 119.

Femke Boekhorst, PR & Communication, ASTRON. Tel.: +31 6 21 23 42 43. E-mail:

Illustration of radio observations of the galaxy IC 2497 with the European VLBI Network (EVN) and the UK’s Multi-Element Radio Linked Interferometer Network MERLIN. The background image is an optical image of Hanny’s Voorwerp and the neighbouring giant galaxy, IC 2497 (Credit: William Keel, Anna Manning, 3.5-m WIYN Telescope), while the magnification glass highlights the associated high-resolution radio observations. Two very compact sources are detected at the heart of IC2497 with the EVN, signalling the presence of an active galactic nucleus there, i.e. an accreting super massive black hole. The surrounding smooth and extended radio emission, arises from a so-called “nuclear starburst” – as the name suggests, this implies that a huge burst of star formation is going on within the central regions of IC 2497. (Copyright: ASTRON).


More information about Hanny’s Voorwerp can be found on:

More information about the European VLBI Network:

More information about MERLIN:


ASTRON is part of NWO, the Netherlands Organisation for Scientific Research.



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