Shell formation in elliptical galaxies

Abstract

Numerical simulations are described which show the formation of shell structures in elliptical/S0 galaxies. The shells are formed as density waves induced in a thick disc population of dynamically cold stars by a weak interaction with another galaxy. The elliptical galaxy is modeled as an oblate spheroidal mass distribution in which test particles are set up on short axis tube orbits with zero radial thickness. The initial particle distribution function is calculated so that the system is in equilibrium. The effect of the primary galaxy ellipticity and the mass of the secondary galaxy on the shell forming process are described. Simulated shell profiles and their radial distribution are also compared with observations. The results support the weak interaction model of shell formation in elliptical/S0 galaxies, and imply that shell galaxies are similar oblate spheroids which appear as type 1 (aligned) systems when viewed edge-on, and as type 2 (all-round) when viewed face-on. The weak interaction model of shell formation is shown to give a more accurate and robust description of the observed phenomena than does the merger model.