Revealing Hanny's Voorwerp: radio observations of IC 2497

Abstract

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 × 10 21 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.

Hanny's Voorwerp

“…Obscuring material in the direction of the core of IC 2497 is clearly present, while the extended continuum implies that a large-scale radio jet is present. These radio observations support an alternative to the light-echo scenario, namely that IC 2497 contains an obscured active galactic nucleus (AGN) with a weak, large-scale radio jet pointing in the direction of Hanny's Voorwerp, perpendicular to the major axis of the galaxy (Józsa et al 2009). Hence, another interpretation of Hanny's Voorwerp is that it is a rare and spectacular example of ongoing AGN feedback in which an obscured AGN at the centre of a relatively nearby galaxy is observed ionising the surrounding IGM (Józsa et al 2009).…”

“…The image shows two compact radio components, separated by ∼300 milliarcsec (∼312 pc) 2 . The component to the southwest (hereafter C1) was initially detected by a short track 18 cm EVN (e-VLBI) observation (Józsa et al 2009), and to the northeast of C1 is a fainter, newly discovered component (hereafter C2).…”

“…Hanny's Voorwerp (SDSS J094103.80+344334.2) 1 is an irregular gas cloud located ∼25 kpc to the southeast of the massive disk galaxy IC 2497 (Lintott et al 2009;Józsa et al 2009). In the optical, the Voorwerp's appearance is dominated by [O III] emission lines, and its spectrum shows strong line emission, with high-ionisation lines (He II, [Ne V]) co-extensive with the continuum (Lintott et al 2009).…”

“…Paradoxically, there is no evidence of an ionising source in the immediate proximity of this nebulosity. Its quiescent kinematics, derived from optical spectra, imply that photoionisation is the predominant ionisation process, rather than ionisation via shocks (Lintott et al 2009;Józsa et al 2009). The original explanation of this phenomenon was provided by Lintott et al (2009), who argued that Hanny's Voorwerp may be the first example of a quasar light echo.…”

“…Radio observations using the Westerbork Synthesis Telescope (WSRT) and the EVN (using the e-VLBI technique) (Józsa et al 2009) have detected a radio continuum source at Table 2 is only available in electronic form at http://www.aanda.org 1 "Voorwerp" is Dutch for object. the central position of IC 2497, and weak, large-scale emission pointing in the direction of Hanny's Voorwerp.…”