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 4 × 10^5 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 5.6 × 10^9 {M}_⊙ 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 2.9 × 10^9 {M}_⊙. Our data hint at a physical connection between the complexes. We also detect H I 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 H I kinematics do not fit regular rotation, thus the cloud complex around IC 2497 is probably of tidal origin. From the H I absorption against the central source, we derive a lower limit of 2.8 ± 0.4 × 10²¹ atoms cm^-2 to the H I 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.