Multi-method-modeling of interacting galaxies I. A unique scenario for NGC4449?

NGC 4449 is an active star-forming dwarf galaxy of Magellanic type. From ra dio observations, van Woerden et al. (1975) found an extended HI-halo aroun d NGC 4449 which is at least a factor of 10 larger than the optical diamete r D-25 approximate to 5.6 kpc. Recently, Hunter et al. (1998) discerned det ails in the HI-halo: a disc-like feature around the center of NGC 4449 and a lopsided arm structure. We combined several N-body methods in order to in vestigate the interaction scenario between NGC 4449 and DDO 125. a close co mpanion in projected space. In a first step fast restricted N-body models a re used to confine a region in parameter space reproducing the main observa tional features. In a second step a genetic algorithm is applied for a uniq ueness test of our preferred parameter set. We show that our genetic algori thm reliably recovers orbital parameters, provided that the data are suffic iently accurate, i.e. all the key features are included. In the third step the results of the restricted N-body models are compared with self-consiste nt N-body simulations. In the case of NGC 4449, the applicability of the si mple restricted N-body calculations is demonstrated. additionally, it is sh own that the HI gas can be modeled here by a purely stellar dynamical appro ach. In a series of simulations, we demonstrate that tile observed features of the extended HI disc can be explained by a gravitational interaction be tween NGC 4449 and DDO 125. According to these calculations the closest app roach between both galaxies happened similar to4 - 6 10(8) yr ago at a mini mum distance of similar to 25 kpc on a parabolic or slightly elliptic orbit . In the case of an encounter scenario, the dynamical mass of DDO 125 shoul d not be smaller than 10% of NGC 4449's mass. Before the encounter, the obs erved HI gas was arranged in a disc with a radius of 35-40 kpc around the c enter of NGC 4449. It had the same orientation as the central ellipsoidal H I structure. The origin of this disc is still unclear, but it might have be en caused by a previous interaction.