Relic jet activity in ‘Hanny’s Voorwerp’ revealed by the LOFAR two metre sky survey

Abstract

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-ionized region in the environs of the galaxy IC2497, first discovered by the Galaxy Zoo project. The new 150 MHz 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 [O iii] 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 150 MHz data, alongside newly-processed archival 1.64 GHz eVLA data, reveal that the extended emission has a steep spectrum, implying an age >108 yr. The jet supplying the extended 150 MHz 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.