  Gas-poor galaxies can be modelled as composite collisionless stellar systems,
with a dark matter halo and one or more stellar components, representing
different stellar populations. The dynamical evolution of such composite
systems is often studied with numerical N-body simulations, whose initial
conditions typically require realizations with particles of stationary galaxy
models. We present a novel method to conceive these N-body realizations, which
allows one to exploit at best a collisionless N-body simulation that follows
their evolution. The method is based on the use of an effective N-body model of
a composite system, which is in fact realized as a one-component system of
particles that is interpreted a posteriori as a multi-component system, by
assigning in post-processing fractions of each particle's mass to different
components. Examples of astrophysical applications are N-body simulations that
aim to reproduce the observed properties of interacting galaxies, satellite
galaxies and stellar streams. As a case study we apply our method to an N-body
simulation of tidal stripping of a two-component (dark matter and stars)
satellite dwarf galaxy orbiting in the gravitational potential of the Milky
Way.
