Chaos in Barred Galaxy Models

We investigate the regular and chaotic motion in a model potential found using the recent developments of the Inverse Problem of Dynamics. The potential describes the motion in the central parts of a barred galaxy. In the absence of rotation chaotic motion is observed when the perturbation strength is near the escape perturbation for a fixed value of the energy. In the rotating cases one observes that the area of chaotic motion on the surface of section decreases as the angular velocity Ω increases and finally all orbits become regular. The character of motion is also checked by computing the Liapunov characteristic exponents in all cases. This revised version was published online in July 2006 with corrections to the Cover Date.     

Response of a Disk Galaxy to a Satellite Galaxy in Elliptical Orbit

We use a 2-dimensional self consistent N-body simulation code in order to investigate the evolution of spiral structure in a disk galaxy caused by one small companion galaxy orbiting in elliptical orbit around the main disk galaxy. In all cases one can see spiral arms forming in the disk of the main galaxy. Our numerical results suggest that there is a connection between the shape of the spiral arms and the eccentricity of the companion's orbit. We also examine the maximum density distribution on the spiral arms and the influence of the companion on the velocity RMS of the stars that form the disk of the main galaxy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chaos in a quasar model with a disk and a massive nucleus

We present a quasar model with a rotating disk and a massive nucleus. We use this model in order to characterize the motion in the model (regular or chaotic) and to connect the extent of the chaotic regions to the physical parameters of the model. Numerical experiments suggest that, there are connections between the extent of the chaotic areas and the parameters of the system. Furthermore, it is shown that the form of numerically found relationships can be expressed analytically. Comparison to previous work is also made. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The structure of motion in a 4-component galaxy mass model

We use a composite galaxy model consisting of a disk-halo, bulge, nucleus and dark-halo components in order to investigate the motion of stars in ther-z plane. It is observed that high angular momentum stars move in regular orbits. The majority of orbits are box orbits. There are also banana-like orbits. For a given value of energy, only a fraction of the low angular momentum stars — those going near the nucleus — show chaotic motion while the rest move in regular orbits. Again one observes the above two kinds of orbits. In addition to the above one can also see orbits with the characteristics of the 2/3 and 3/4 resonance. It is also shown that, in the absence of the bulge component, the area of chaotic motion in the surface of section increases, significantly. This suggests that a larger number of low angular momentum stars are in chaotic orbits in galaxies with massive nuclei and no bulge components.     

A new dynamical spectrum for galactic potentials

We investigate the properties of motion, using the distribution of the values of a new dynamical parameter, in two different galactic potentials. The first is a potential made up of harmonic oscillators while the second is a logarithmic potential. We call the distribution functions of the new parameter the S(r) dynamical spectrum. A comparison between the spectrum of stretching numbers S(α) and the new spectrum is made. The results of our numerical calculations suggest that the S(r) spectrum is a better discriminant between different types of orbits while requiring considerable less computation time. An application of the new dynamical spectrum to barred galaxy models is also presented.     

Determining the character of motion in quiet and active galaxies with a satellite companion

A galaxy model with a satellite companion is used to study the character of motion for stars moving in the x ‐y plane. It is observed that a large part of the phase plane is covered by chaotic orbits. The percentage of chaotic orbits increases when the galaxy has a dense nucleus of massM_n. The presence of the dense nucleus also increases the stellar velocities near the center of the galaxy. For small values of the distance R between the two bodies, low energy stars display a chaotic region near the centre of the galaxy, when the dense nucleus is present, while for larger values of R the motion in active galaxies is regular for low energy stars. Our results suggest that in galaxies with a satellite companion, the chaotic character of motion is not only a result of galactic interaction but also a result caused by the dense nucleus. Theoretical arguments are used to support the numerical outcomes. We follow the evolution of the galaxy, as mass is transported adiabatically from the disk to the nucleus. Our numerical results are in satisfactory agreement with observational data from M51‐type binary galaxies (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Bifurcations of periodic orbits in a rotating galaxy

The characteristics of several families of periodic orbits are investigated numerically in a time-independent, bisymmetrical dynamical system which is considered to describe the motion on the plane of rotation of a nearly axisymmetric galaxy. The results of the present study are compared with those found by Caranicolas and Barbanis (1982).     

Resonant periodic orbits in a bisymmetrical potential

In the present paper we discuss the properties of several families of resonant periodic orbits in a general, time-independent, two-dimensional potential field, symmetric with respect to both axesx andyy. We classify the different cases by varying the parameters of the problem. In order to verify the theoretical results we also present some numerical examples.     

Orbital characteristics of dynamical models of elliptical galaxies

We use three different dynamical models for describing the motion in the meridian plane of an elliptical galaxy. Orbital characteristics of two polynomial models are compared with those given by a logarithmic potential for a fixed axial ratio of the equipotential surfaces, We also study the effects of an asymmetric perturbation caused by a companion galaxy on the orbital behaviour of the above models. Finally we present some theoretical arguments in order to support the numerical results.