Letter to the Editor

Hanny's Voorwerp ^*

Evidence of AGN activity and a nuclear starburst in the central regions of IC 2497

¹ Joint Institute for VLBI in Europe (JIVE), Postbus 2, 7990 AA Dwingeloo, The Netherlands
² Leiden Observatory, Leiden University, PO Box 9513, 2300RA Leiden, The Netherlands
³ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, Univ. Manchester, Alan Turing Building, Oxford Road, Manchester, M13 9PL, UK e-mail: hayden.rampadarath@manchester.ac.uk
⁴ Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands e-mail: garrett@astron.nl
⁵ Centre for Supercomputing, Swinburne Univ. of Technology, Mail number H39, PO Box 218, Hawthorn, Victoria 3122, Australia
⁶ Kapteyn Astronomical Institute, Univ. Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
⁷ MTA Research Group for Physical Geodesy and Geodynamics, PO Box 91, 1521 Budapest, Hungary
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 Einstein Fellow

Received: 13 April 2010
Accepted: 17 June 2010

Abstract

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 10⁵ 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 > 5 ) of 12.4 /yr. These results are in line with the hypothesis that the ionisation nebula “Hanny's Voorwerp” at a distance of ~ 15–25 kpc from the galaxy is ionised by the radiation cone of the AGN.