The Density Profiles of Tidally Stripped Galaxies

A simple model for collision-induced changes in the structure of a nonrotating spherical galaxy is presented, and its predictions are compared with N-body simulations. The numerical techniques, the evolution of the density profiles immediately after an encounter, and the relaxed state approached at late times are described. The De Vaucouleurs profiles are found to be very robust. Models whose initial surface densities are described by an r exp 1/4 law are still well fitted by such a law after collisions in which the target galaxy loses up to 40 percent of its mass. The tidal classes defined by Kormendy (1977) may be interpreted as a transient phenomenon produced by the tidal mixing of stars onto less bound orbits. The departure from an r exp 1/4 law evolves on the local dynamical time scale, and its radial position can be used to estimate the time since the tidal encounter. Although tidal encounters do not truncate density profiles, the isophotal radii of galaxies usually shrink.