Letter to the Editor

Revealing Hanny's Voorwerp: radio observations of IC 2497

¹ Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands e-mail: jozsa@astron.nl
² Leiden Observatory, Univ. Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Centre for Astrophysics and Supercomputing, Univ. Swinburne, Mail number H39, Swinburne University of Technology, PO Box 218, Hawthorn, Victoria 3122, Australia
⁴ Kapteyn Astronomical Institute, Univ. Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
⁵ Joint Institute for VLBI in Europe, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁶ MTA Research Group for Physical Geodesy and Geodynamics, PO Box 91, 1521 Budapest, Hungary
⁷ Univ. Oxford, Dept. Physics, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
⁸ Univ. Alabama, Dept. Physics & Astronomy, Box 870324, University of Alabama, Tuscaloosa, AL 35487-0324, USA
⁹ Univ. Yale, Dept. Physics, J.W. Gibbs Laboratory, 260 Whitney Avenue, Yale University, New Haven, CT 06511, USA
¹⁰ Institute of Cosmology and Gravitation, Dennis Sciama Building, Burnaby Road, Portsmouth, PO1 3FX, UK

Received: 29 April 2009
Accepted: 12 May 2009

Abstract

We present multi-wavelength radio observations in the direction of the spiral galaxy IC 2497 and the neighbouring emission nebula known as “Hanny's Voorwerp”. Our WSRT continuum observations at and reveal the presence of extended emission at the position of the nebulosity, although the bulk of the emission remains unresolved at the centre of the galaxy. e-VLBI observations show that on the milliarcsecond-scale, a faint central compact source is present in IC 2497 with a brightness temperature in excess of 4  . With the WSRT, we detect a large reservoir of neutral hydrogen in the proximity of IC 2497. One cloud complex with a total mass of 5.6  to the South of IC 2497 encompasses Hanny's Voorwerp. Another cloud complex is located at the position of a small galaxy group to the West of IC 2497 with a mass of 2.9  . Our data hint at a physical connection between the complexes. We also detect in absorption against the central continuum source of IC 2497. Our observations strongly support the hypothesis that Hanny's Voorwerp is being ionised by an AGN in the centre of IC 2497. In this scenario, a plasma jet associated with the AGN clears a path through the ISM/IGM in the direction of the nebulosity. The large-scale radio continuum emission possibly originates from the interaction between this jet and the large cloud complex in which Hanny's Voorwerp is embedded. The  kinematics do not fit regular rotation, thus the cloud complex around IC 2497 is probably of tidal origin. From the  absorption against the central source, we derive a lower limit of 2.8 ± 0.4  to the  column density. However, assuming non-standard conditions for the detected gas, we cannot exclude the possibility that the AGN in the centre of IC 2497 is Compton-thick.