Minor Mergers and the Formation of Shell Galaxies

We present a new mechanism which can explain the observed radial displacement between the stellar and gaseous shells observed around early type galaxies. Our model is based on the well-known phase-wrapping process taking into account that a fraction of the interstellar medium is clumpy and consequently it experiences a low dynamical friction during a merging event. Moreover, our recent detection of cold molecular gas associated with the outer shells of Centaurus A is in agreement with the proposed model. This suggests that a considerable fraction of molecular gas in minor mergers may be distributed away from the nuclear regions and remain undetected.