Revealing Hanny's Voorwerp: radio observations of IC 2497
  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 1.4 GHz and 4.9 GHz, 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
1.65 GHz observations show that on the milliarcsecond-scale a faint central
compact source is present in IC 2497 with a brightness temperature in excess of
4E5 K. 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 5.6E9 Msol to the
South of IC 2497, encompasses Hanny's Voorwerp. Another cloud complex is
located at the position of a small galaxy group ~100 kpc to the West of IC 2497
with a mass of 2.9E9 Msol. Our data hint at a physical connection between both
complexes. We also detect HI 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 towards the nebulosity. The large-scale radio continuum emission
possibly originates from the interaction between this jet and the large cloud
complex that Hanny's Voorwerp is embedded in. The HI kinematics do not fit
regular rotation, thus the cloud complex around IC 2497 is probably of tidal
origin. From the HI absorption against the central source, we derive a lower
limit of 2.8E21 +- 0.4E21 atoms/sqcm to the HI 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.
