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We present the detection of a low surface-brightness stellar counterpart toan enormous (190 kpc) ring of neutral hydrogen (HI) gas that surrounds thenearby radio galaxy B2 0648+27. This system is currently in an evolutionarystage between major merger and (radio-loud) early-type galaxy. In a previouspaper we investigated in detail the timescales between merger, starburst andAGN activity in B2 0648+27, based on its unusual multi-wavelength properties(large-scale HI ring, dominating post-starburst stellar population andinfra-red luminosity). In this Research Note we present deep optical B- andV-band imaging that provides further evidence for the merger origin of B20648+27. The host galaxy shows a distorted optical morphology and a broad tidalarm is clearly present. A low surface-brightness stellar tail or partial ringcurls around more than half the host galaxy at a distance of up to 55 kpc fromthe centre of the galaxy, following the large-scale, ring-like HI structurethat we detected previously around this system. The gas and stars in this ringare most likely tidally expelled material that slowly fell back onto the hostgalaxy after the merger event. There also appear to be sites of star formationin the HI ring that may have formed within the gaseous tidal debris after themerger. We argue that the observed properties of the gas and stars in B20648+27, as well as the apparent time-delay between the merger and thestarburst event, may be the logical result of a merger between two gas-richdisk galaxies with a prominent bulge, or of a merger between an elliptical anda gas-rich spiral galaxy. There also appears to be a significant time-delaybetween the merger/starburst event and the current episode of radio-AGNactivity.

From major merger to radio galaxy: low surface-brightness stellar counterpart to the giant HI ring around B2 0648+27