Modeling Encounters of Galaxies: The Case of NGC4449

TL;DR

This paper develops a combined N-body and genetic algorithm method to determine interaction parameters of galaxies, successfully applied to NGC4449, revealing insights into its HI distribution and interaction history.

Contribution

It introduces an automated, combined approach using restricted N-body models and genetic algorithms for analyzing galaxy encounters, demonstrating its effectiveness on NGC4449.

Findings

Genetic algorithm reliably recovers orbital parameters.

Restricted N-body models are applicable to NGC4449.

Interaction with DDO125 explains main HI features.

Abstract

(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.