Research Note

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

¹ Department of Astronomy, Columbia University, Mail Code 5246, 550 West 120th Street, New York, N.Y. 10027, USA e-mail: emonts@astro.columbia.edu
² Netherlands Foundation for Research in Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ Steward Observatory, Department of Astronomy, The University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁵ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK

Received: 24 April 2008
Accepted: 13 June 2008

Abstract

We present the detection of a low surface-brightness stellar counterpart to an enormous (190 kpc) ring of neutral hydrogen (H I) gas that surrounds the nearby radio galaxy B2 0648+27. This system is currently in an evolutionary stage between major merger and (radio-loud) early-type galaxy. In a previous paper we investigated in detail the timescales between merger, starburst and AGN activity in B2 0648+27, based on its unusual multi-wavelength properties (large-scale H I ring, dominating post-starburst stellar population and infra-red luminosity). In this Research Note we present deep optical B- and V-band imaging that provides further evidence for the merger origin of B2 0648+27. The host galaxy shows a distorted optical morphology and a broad tidal arm is clearly present. A low surface-brightness stellar tail or partial ring curls around more than half the host galaxy at a distance of up to 55 kpc from the centre of the galaxy, following the large-scale, ring-like H I structure that we detected previously around this system. The gas and stars in this ring are most likely tidally expelled material that slowly fell back onto the host galaxy after the merger event. There also appear to be sites of star formation within the H I ring that may have formed within the gaseous tidal debris after the merger. We argue that the observed properties of the gas and stars in B2 0648+27, as well as the apparent time-delay between the merger and the starburst event, may be the logical result of a merger between two gas-rich disk galaxies with a prominent bulge, or of a merger between an elliptical and a gas-rich spiral galaxy. There also appears to be a significant time-delay between the merger/starburst event and the current episode of radio-AGN activity.