#shell galaxies

What's happening to galaxy NGC 474? The multiple layers of emission appear strangely complex and unexpected given the relatively featureless appearance of the elliptical galaxy in less deep images. The cause of the shells is currently unknown, but possibly tidal tails related to debris left over from absorbing numerous small galaxies in the past billion years. Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy just above NGC 474 is causing density waves to ripple though the galactic giant. Regardless of the actual cause, the above image dramatically highlights the increasing consensus that at least some elliptical galaxies have formed in the recent past, and that the outer halos of most large galaxies are not really smooth but have complexities induced by frequent interactions with -- and accretions of -- smaller nearby galaxies. The halo of our own Milky Way Galaxy is one example of such unexpected complexity. NGC 474 spans about 250,000 light years and lies about 100 million light years distant toward the constellation of the Fish (Pisces).

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 and the interaction model 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.