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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., Comment: Paper presented at the 10th EVN Symposium in Manchester, Sep. 2010

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. We detect two compact radio sources, with brightness temperatures above 10e5 K, suggesting that they are related to AGN activity. We show that the total 18 cm radio emission from the galaxy is dominated neither by these compact sources nor large-scale emission, but extended emission confined within a sub-kpc central region. IC 2497 therefore appears as a typical luminous infrared galaxy that exhibits a nuclear starburst with a massive star formation rate (M > 5M_solar) of 12.4 M_solar/yr. These results are in line with the hypothesis that the ionisation nebula "Hanny's Voorwerp" at a distance of approx. 15-25 kpc from the galaxy is ionised by the radiation cone of the AGN., Comment: 5 pages, 3 figures, A&A letter accepted on 17/06/2010

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., Comment: 4 pages, 5 figures, A&A, letter to the editor, accepted for publication