Formation of shell galaxies. I - Spherical potentials

Abstract

The formation of shells through interactions of low-mass systems with spherical galaxies is investigated using the restricted three-body method. It is shown that such encounters lead to the development of sharp-edged features under a much wider set of conditions than considered by Quinn. In particular: (1) collisions involving low-mass spheroids, in addition to low-mass disks, can produce shells and would naturally account for the absence of gas and dust in some elliptical shell galaxies; (2) shells can be formed from companions on nonradial orbits, as well as radial orbits, and tend to be rather confused, representative of the majority of observed shell galaxies; and (3) mass transfer that occurs during roughly parabolic encounters can also generate shells. Shells appear to be a general consequence of interactions of spatially compact and/or dynamically cold systems with large galaxies. The variety of structures exhibited by the present simulations resembles the observed shell galaxies. However, owing to the number of free parameters and the chaotic nature of most observed shell systems, it seems unlikely in retrospect that shells will prove widely useful as diagnostics of the potential of the host galaxies. Possible exceptions remain such unusually regular systems as NGC 3923.