ABSTRACT

N-body simulations of M51-system suggest that the companion is currently moving in a highly inclined (75°) but bound orbit (e = 0.25) with respect to the main system. In this model the inner spiral structure as well as the extended outer tail follow from excitation during an earlier disk plane crossing about 400 million years ago, while the most recent crossing occurred less than 100 million years ago.

INTRODUCTION

The M51 system (NGC 5194/5) is an expectionally well observed spiral galaxy with a clear grand design pattern. Several attempts have been made to explain its structure with N-body modelling, starting with the classic test-particle simulations of Toomre and Toomre (1972). Although these and the later self-gravitating models of Hernquist (1990) have been quite successful in reproducing the tidal bridge and tail structures, they usually predict such a short interval since the principal perturbation at disk plane crossing, that it is hard to explain the existence of the strong inner spiral pattern. Moreover, the arm kinks going from the inner structure to the bridge and tail arms strongly suggest a more complicated process. Finally, the recently observed (Rots et al. 1990) large extent hydrogen “far tail” requires serious revisions in existing simulation models. Howard and Byrd (1990) first suggested that all these features could be simultaneously accounted for by assuming a bound companion orbit, with several strong perturbation events at successive disk plane crossing.

N–BODY MODEL

In the present study the multiple encounter model is refined and studied with a fully 3-dimensional self-gravitating code, similar to that used in modeling of Arp 86 pair (Salo and Laurikainen 1993).