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Hanny’s Voorwerp: a nuclear starburst in IC2497
M. A. Garrett1-3 1ASTRON, Netherlands Institute for Radio Astronomy, Post box 2, 7990AA, Dwingeloo, The Netherlands. 2Leiden Observatory, Leiden University, Post box 9513, 2300RA Leiden, The Netherlands. 3Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Australia. E-mail: garrett@astron.nl H. Rampadarath Curtin University of Technology, GPO BOX U1987, Perth, WA 6845, Australia. E-mail: Hayden.Rampadarath@icrar.org G. I. G. Józsa ASTRON, Netherlands Institute for Radio Astronomy, Postbus 2, 7990AA, Dwingeloo, The Netherlands. E-mail: jozsa@astron.nl T. W. B. Muxlow Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, Univ. Manchester, Alan Turing Building, Oxford Road, Manchester, M13 9PL, UK. E-mail: Tom.Muxlow@manchester.ac.uk T. A. Oosterloo ASTRON, Netherlands Institute for Radio Astronomy, Postbus 2, 7990AA, Dwingeloo, The Netherlands. Kapteyn Astronomical Institute, Univ. Groningen, Postbus 800, 9700 AV Groningen, The Netherlands. E-mail: oosterloo@astron.nl Z. Paragi JIVE, Joint Institute for VLBI in Europe, Postbus 2, 7990AA, Dwingeloo, The Netherlands. MTA Research Group for Physical Geodesy and Geodynamics, PO Box 91, 1521 Budapest, Hungary. E-mail: paragi@jive.nl R. Beswick Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, Univ. Manchester, Alan Turing Building, Oxford Road, Manchester, M13 9PL, UK. E-mail: Robert.Besick@manchester.ac.uk 10th European VLBI Network Symposium and EVN Users Meeting: VLBI and the new generation of radio arrays Manchester, UK September 20-24, 2010

Hanny’s Voorwerp: a nuclear starburst in IC2497 Michael A. Garrett
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H. van Arkel ASTRON, Netherlands Institute for Radio Astronomy, Postbus 2, 7990AA, Dwingeloo, The Netherlands. E-mail: hanny.van.arkel@home.nl K. Schawinski Univ. Yale, Dept. Physics, J.W. Gibbs Laboratory, 260 Whitney Avenue, Yale University, New Haven, CT 06511, USA. E-mail: kevin.schawinski@yale.edu W. C. Keel Univ. Alabama, Dept. Physics & Astronomy, Box 870324, University of Alabama, Tuscaloosa, AL 35487-0324, USA. E-mail: wkeel@bama.ua.edu We present high and intermediate resolution radio observations of the central region in the spiral galaxy IC 2497, performed using the European VLBI Network (EVN) at 18 cm, and the Multi-Element Radio Linked Interferometer Network (MERLIN) at 18 cm and 6 cm. The e-VLBI observations detect two compact radio sources with brightness temperatures in excess of 105 K, suggesting that they are associated with an AGN located at the centre of the galaxy. We show that IC2497 lies on the FIR-radio correlation and that the dominant component of the 18 cm radio flux density of the galaxy is associated with extended emission confined to sub-kpc scales. IC 2497 therefore appears to be a luminous infrared galaxy that exhibits a nuclear starburst with a total star formation rate (assuming a Salpeter IMF) of ~ 70 M⊙/yr. Typically, vigorous star forming galaxies like IC2497 always show high levels of extinction towards their nuclear regions. The new results are in-line with the hypothesis that the ionisation nebula “Hanny’s Voorwerp”, located ∼15−25 kpc from the galaxy is part of a massive gas reservoir that is ionised by the radiation cone of an AGN that is otherwise obscured along the observer’s line-of-sight. 1.Introduction
Hanny’s Voorwerp (SDSS J094103.80+344334.2) is an irregular gas cloud located ∼ 25 kpc to the southeast of the massive disk galaxy IC 2497 [1]. In the optical, the Voorwerp’s appearance is dominated by [O III] emission lines, and its spectrum shows strong line emission, with high-ionisation lines co-extensive with the continuum. Paradoxically, there is no clear evidence of an ionising source in the immediate proximity of this nebulosity. The original interpretation of this phenomenon [1], suggested that Hanny’s Voorwerp may be the first example of a quasar light echo. Indeed, Lintott et al. proposed that in the past, the central luminosity of IC2497 approached quasar-like levels but that around 105 years ago this decreased to the lower-levels of activity observed today. X-ray observations [2] are also interpreted in this context – there it is concluded that the galaxy’s central engine has decreased its radiative output by at least 2 orders of magnitude within the last 70,000 years.IC 2497 is a quasar that has transitioned from a high state to a radiatively inefficient state where the bulk of the energy is dissipated not as radiation but as either thermal or kinetic energy. Radio observations using the Westerbork Synthesis Telescope (WSRT) and the European VLBI Network (EVN) have detected a radio continuum source at the central position of IC 2497, and