Relic jet activity in "Hanny's Voorwerp" revealed by the LOFAR Two metre Sky Survey

TL;DR

This study uses LOFAR 150MHz radio observations to uncover relic jet activity in Hanny's Voorwerp, revealing extended emission and providing insights into the history of AGN activity in galaxy IC2497.

Contribution

First detailed low-frequency radio imaging of Hanny's Voorwerp showing relic jet structures and their implications for galaxy IC2497's AGN history.

Findings

Detection of large-scale extended radio emission from IC2497

Evidence of a steep spectrum indicating an age >10^8 years

Jet activity ceased long before recent X-ray luminosity changes

Abstract

We report new observations of "Hanny's Voorwerp" (hereafter HV) taken from the second data release of the LOFAR Two-metre Sky Survey (LoTSS). HV is a highly-ionised region in the environs of the galaxy IC2497, first discovered by the Galaxy Zoo project. The new 150MHz observations are considered in the context of existing multi-frequency radio data and archival narrow-band imaging from the Hubble Space Telescope, centred on the [Oiii] emission line. The combined sensitivity and spatial resolution of the LoTSS data -- which far exceed what was previously available at radio frequencies -- reveal clear evidence for large-scale extended emission emanating from the nucleus of IC2497. The radio jet appears to have punched a hole in the neutral gas halo, in a region co-located with HV. The new 150MHz data, alongside newly-processed archival 1.64GHz eVLA data, reveal that the extended emission has a steep spectrum, implying an age $>10^8$yr. The jet supplying the extended 150MHz structure must have "turned off" long before the change in X-ray luminosity reported in recent works. In this picture, a combination of jet activity and the influence of the radiatively efficient active galactic nucleus are responsible for the unusual appearance of HV.