Modeling Encounters of Galaxies: The Case of NGC4449

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 the high-dimensional 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, provided that the data are sufficiently accurate, i.e. all the key features are included. 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. Additionally, it is shown that the HI gas can be modeled here without any restrictions by a purely stellar dynamical approach. 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 D25 ≈ 5.6 kpc. 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 another nearby dwarf galaxy, DDO125. According to these calculations the closest approach between both galaxies happened ∼ 3.5 · 10 yr ago at a minimum distance of ∼ 25 kpc on a nearly parabolic orbit. In case of an encounter scenario, the dynamical mass of DDO125 should not be smaller than 10% of NGC4449’s mass. Before the encounter the observed HI was arranged in a disc with a radius of 35–40 kpc around the center of NGC4449 which had the same orientation as the central ellipsoidal HI structure. The origin of this disc is still unclear, but it might be caused by a previous interaction.