Hanny's Voorwerp: a nuclear starburst in IC2497

TL;DR

This paper reports high-resolution radio observations of IC 2497, revealing a nuclear starburst and an obscured AGN, and discusses the ionisation of Hanny's Voorwerp by the AGN's radiation cone.

Contribution

It provides new radio imaging evidence of a nuclear starburst and an obscured AGN in IC 2497, supporting the ionisation of Hanny's Voorwerp by AGN radiation.

Findings

IC 2497 hosts a nuclear starburst with ~70 M*/yr star formation rate.

The galaxy contains an obscured AGN with compact radio sources.

Hanny's Voorwerp is ionised by the AGN's radiation cone.

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. 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.