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Radio observations of Hanny's Voorwerp and IC 2497

Submitter: G.I.G. Jozsa, M.A. Garrett, T.A. Oosterloo, H. Rampadarath
Description: Hanny's Voorwerp, green on our image, was discovered by Hanny van Arkel (we reported about her visit at ASTRON previously). The Voorwerp is a huge, very hot gas cloud that does not contain any apparent heating source (like stars).

Our WSRT radio continuum observations (white contours) show a source at the centre of IC 2497 with a jet-like extension in the direction of The Voorwerp. The e-VLBI observations reveal a very small source at the centre of the galaxy, less than 0.1 light years across (top-right image). This suggests that IC 2497 contains a supermassive black hole at its centre, emitting radiation in the process of swallowing gas. A plasma jet, emanating from the black hole, clears a channel through the thick dust and gas that surrounds the black hole, permitting the intense radiation to reach Hanny's Voorwerp. In the radio observations we see the central black hole and the plasma jet hitting the gas.

WSRT observations of the neutral hydrogen show that Hanny's Voorwerp is embedded in a gas cloud of 5000 million solar masses (red in the image), which has an extension stretching out to the position of a galaxy group (to the right). Hydrogen spectra reveal the connection between the gas around IC 2497 and the galaxy group (bottom picture). The Voorwerp is part of a much bigger gas cloud, the remnant of an interaction between IC 2497 and a galaxy group.

Finally, we detect neutral hydrogen in absorption (picture top left), as a shadow against the central continuum source. This indicates that there could be enough material in front of IC 2497 to block the view towards the central regions of this galaxy at other wavelengths.
Copyright: Credits: ASTRON, optical image based on SDSS data
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