Shell Formation in NGC474

We present broad band optical (B, R, I) and H alpha images of the classic shell galaxy NGC 474 (Arp 227), and computer simulations of the shell forming process. We have selected NGC 474 as it has shells of unusually high surface brightness and it has a nearby interacting spiral companion, NGC 470. The merger model (Quinn 1984; Dupraz & Combes 1987; Hernquist & Quinn 1988, 1989) and the interaction model (Thomson & Wright 1990; Thomson 1991) can explain various, but not all aspects of observed shell structure. According to the merger model, shells form as the result of a collision with another secondary elliptical or disk galaxy. The shells form from a phase wrapping of the disrupted secondary galaxy in the fixed potential well of the primary elliptical galaxy. According to the interaction model, the shells form as density waves in a dynamically cold component (thick disk) during a flyby interaction with another galaxy. Schombert & Wallin (1987) originally proposed an interaction model for the formation of the shells in NGC 474, based on the shell colours and close proximity of NGC 470. We compare our new, high S/N imaging data, with computer simulations of both the merger and interaction models to determine the most likely formation mechanism for the shells in NGC 474.