Search

We propose a theory to explain the formation of spiral arms and of all types of outer rings in barred galaxies, extending and applying the technique used in celestial mechanics to compute transfer orbits. Thus, our theory is based on the chaotic orbital motion driven by the invariant manifolds associated to the periodic orbits around the hyperbolic equilibrium points. In particular, spiral arms and outer rings are related to the presence of heteroclinic or homoclinic orbits. Thus, R1 rings are associated to the presence of heteroclinic orbits, while R1R2 rings are associated to the presence of homoclinic orbits. Spiral arms and R2 rings, however, appear when there exist neither heteroclinic nor homoclinic orbits. We examine the parameter space of three realistic, yet simple, barred galaxy models and discuss the formation of the different morphologies according to the properties of the galaxy model. The different morphologies arise from differences in the dynamical parameters of the galaxy., Comment: 8 pages, 4 figures, in the proceedings of the conference: "Chaos in Astronomy", Athens, September 2007, G. Contopoulos and P.A. Patsis (eds), to be published by Springer

We present N-body simulations of groups of galaxies with a number of very different initial conditions. These include spherical isotropic, nonspherical anisotropic collapses and virialised spherical systems. In all cases but one the merging instability leads to the formation of a giant central galaxy in the center of the group. The initial conditions of the exception are such that no galaxies are present in the central part of the group. Thus some central seed of material is necessary to trigger the formation of a giant central galaxy. We concentrate on the properties of these giant central galaxies. Spherical virialised systems give rise to relatively round and isotropic systems, while aspherical initial conditions give rise to triaxial objects with anisotropic velocity dispersion tensors. In the latter cases the orientation of the resulting central galaxy is well correlated with that of the initial cluster. We compare the projected properties of the objects formed with the properties of real brightest cluster member galaxies. The surface density profiles are in good agreement with the observed surface brightness profiles. In the case of extended virialised groups the projected properties of the giant central galaxy are the same as the properties of cD galaxies. These include a halo of luminous material and a nearly flat velocity dispersion profile., Comment: 19 pages, 21 Postscript figures, Latex version. Accepted in Astronomy & Astrophysics main Journal

In this paper we test the performance of explicit simulations of groups of galaxies, (i.e. simulations in which each galaxy is treated as a mass point and the physics of the interactions is modelled by specific analand the physics of the interactions is modelled by specific analytical prescriptions for merging conditions), by comparing them with fully self-consistent simulations starting from identical initial conditions. The quality of the explicit simulations is very unequal. For somThe quality of the explicit simulations is very unequal. For some prescriptions the results are complete disagreement with the self-consistent simulations. The inclusion of other dynamical effects like dynamical friction gives, in some cases, better agreeement.like dynamical friction gives, in some cases, better agreeement. We also propose a new merging criterion, which, combined with dynamical friction, gives much better agreement with self-consistent simulations in a variety of initial conditions, but even this criterion has a limited ranof initial conditions, but even this criterion has a limited range of applicability., Comment: 17 pages LaTex, 10 Postscript figures, uses a4.sty, a4wide.sty. To appear in A&A