The nuclear ring in the unbarred galaxy NGC 278: Result of a minor merger?

¹ Centre for Astrophysics Research, University of Hertfordshire, Hatfield, Herts AL10 9AB, UK e-mail: j.knapen@star.herts.ac.uk
² University of Nottingham, School of Physics and Astronomy, University Park, Nottingham, NG7 2RD, UK
³ Department of Physics and Astronomy, Cardiff University, 5 The Parade, Cardiff CF24 3YB, UK
⁴ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands

Received: 17 September 2003
Accepted: 24 April 2004

Abstract

We present fully sampled high angular resolution two-dimensional kinematics in the Hα spectral line, optical and near-infrared imaging, as well as 21 cm atomic hydrogen data of the spiral galaxy NGC 278. This is a small non-barred galaxy, which has a bright star forming inner region of about 2 kpc in diameter, reminiscent of nuclear rings seen mainly in barred galaxies. The Hα kinematics show a disturbed velocity field, which may be partly the result of spiral density wave streaming motions. The 21 cm data trace the atomic hydrogen well outside the optical disk. The is not abundant but clearly shows disturbed morphology and kinematics. We postulate that the current structure of NGC 278 is a result of a recent interaction with a small gas-rich galaxy, which is now dispersed into the outer disk of NGC 278. Non-axisymmetries set up in the disk by this minor merger may well be the cause of the intense star formation in the inner region, which can be interpreted as a rare example of a nuclear ring in a non-barred galaxy. Rather than being induced by a bar, this nuclear ring would then be the direct result of an interaction event in the recent history of the galaxy.