Using N-body simulations we study the structures induced on a galactic disc by repeated flybys of a companion in decaying eccentric orbit around the disc .
Our system is composed by a stellar disc , bulge and live dark matter halo , and we study the system ’ s dynamical response to a sequence of a companion ’ s flybys , when we vary i ) the disc ’ s temperature ( parameterized by Toomre ’ s Q-parameter ) and ii ) the companion ’ s mass and initial orbit .
We use a new 3D Cartesian grid code : MAIN ( Mesh-adaptive Approximate Inverse N-body solver ) .
The main features of MAIN are reviewed , with emphasis on the use of a new Symmetric Factored Approximate Sparse Inverse ( SFASI ) matrix in conjunction with the multigrid method that allows the efficient solution of Poisson ’ s equation in three space variables .
We find that : i ) companions need to be assigned initial masses in a rather narrow window of values in order to produce significant and more long-standing non-axisymmetric structures ( bars and spirals ) in the main galaxy ’ s disc by the repeated flyby mechanism .
ii ) a crucial phenomenon is the antagonism between companion-excited and self-excited modes on the disc .
Values of Q > 1.5 are needed in order to allow for the growth of the companion-excited modes to prevail over the the growth of the disc ’ s self-excited modes .
iii ) We give evidence that the companion-induced spiral structure is best represented by a density wave with pattern speed nearly constant in a region extending from the ILR to a radius close to , but inside , corotation .