Orbital Structure of Galaxies in N-Body Simulations
by Roland Jesseit
The orbits on which Stars move in a galaxy are a good indicator for the shape of the mass distribution in this galaxy. Usually the time a star need to complete one orbit cannot be observed in an astronomers life time. If we want to study the complexity of orbital structure, we need to use computer models of galaxy formation. One such possible formation mechanism for elliptical galaxies is the collision of two massive disk galaxies, also sometimes called a major merger (Naab & Burkert 03). We determined numerically the orbital content of the remnants of 112 such merger events. The distribution of orbits varies a lot from remnant to remnant. In Fig.1 we see an example for the orbital decomposition of a remnant with two galaxies of the same mass. The five most important orbit classes are shown from top to bottom 1.) minor axis tubes 2.) inner major axis tubes 3.) outer major axis tubes 4.) box orbits 5.) boxlets. The left part of the figure shows the density distribution of the particles classified for each orbit class for the projection on the three principal planes (xy, xz and yz plane) and the right part shows a single orbit of each class for comparison. 1.) The minor axis tubes revolve around the shortest axis of the galaxy. They can be easily identified by the hole seen in the xy-plane. Oblate disk-like structure, are supported by these orbits. 2.) and 3.) Both major axis tube classes revolve around the longest axis of the galaxy, but the inner major axis tubes have lower angular momentum. The inner major axis tubes are responsible for very prolate-cigar like- shapes, while the outer major axis tubes exist either in prolate or spherical galaxies. 4.) and 5.) Box orbits have no sense of rotation and go through the center of a galaxy. This can be seen in the density plots. Some box orbits are resonant, i. e. their oscillation frequencies at least two directions are integer multiple of a single frequency. They exist also in the outskirts of a remnant, but are not very common.