  (abridged) Several N-body methods were combined in order to develop a method
for the determination of the parameters of interacting galaxies. This method
has been applied to the HI distribution of NGC4449. In a first step the fast
restricted N-body models were used to confine a region in parameter space
reproducing the main observational features. In a second step a genetic
algorithm is introduced which allows for an automated search in parameter space
as well as for a uniqueness test of special parameter sets. Using artificial
data I show that the genetic algorithm reliably recovers orbital parameters. In
the third step the results of the restricted N-body models are compared with
detailed self-consistent N-body simulations. In the case of NGC4449 the
applicability of the simple restricted N-body calculations is demonstrated.
  NGC4449 is an active star-forming dwarf galaxy of Magellanic type. From radio
observations van Woerden et al. (1975) found an extended HI-halo around NGC4449
which is at least a factor of 10 larger than the optical diameter. More
recently, Bajaja et al. (1994) and Hunter et al. (1998) discerned details in
the HI-halo: a disc-like feature around the center of NGC4449 and a lopsided
arm structure. In a series of simulations it is demonstrated that the main
features can be obtained by a gravitational interaction between NGC4449 and
DDO125, another nearby dwarf galaxy. According to these calculations the
closest approach between both galaxies happened 350 Myr ago at a minimum
distance of 25 kpc on a nearly parabolic orbit. In case of the encounter
scenario, the dynamical mass of DDO125 should not be smaller than 10% of
NGC4449's mass. The origin of this disc is still unclear, but it might be
caused by a previous interaction.
