STRUCTURE AND DYNAMICS OF ELLIPTIC GALAXIES

Elliptical galaxies are collisionless stellar systems, where the orbits of the individual stars are determined by the mean gravitational field generated by the stellar system as a whole. Their collective dynamical behaviour presents some formal analogies with that of high-temperature plasmas. Elliptical galaxies had been long thought to be much like the classical ellipsoidal figures of equilibrium, i.e. stellar 'fluids' flattened by rotation. Later on this picture was shown to be inadequate as a result of the first spectroscopic measurements, which indicated that rotation probably plays a secondary role, and of simulations of galaxy formation by collisionless collapse, which produced final equilibrium configurations with mass density distributions similar to those inferred from the data and characterized by anisotropic pressure. Therefore, it now seems that a proper study of elliptical galaxies should proceed through a kinetic (so-called stellar dynamical) description. This article reviews the modelling of elliptical galaxies, centred on the question of their overall mass distribution and intrinsic symmetry and also taking into account the three major issues of stability, dark matter and galaxy formation. We illustrate how some stellar dynamical descriptions can account for the observed luminosity and velocity dispersion profiles, under plausible physical' assumptions suggested by observations and N-body simulations. This modelling process currently faces non-trivial difficulties when dealing with significantly non-spherical systems; on the other hand, integrable self-consistent, stellar dynamical, triaxial models constructed so far apparently do not have a realistic density profile. This review focuses on a few physical and methodological issues, bringing out some interesting connections between the astrophysical problem that is considered and other branches of physics. Reference is given to other review articles that complement the approach taken here.