When satellite galaxies orbit within dark matter halos, they experience tidal forces due to the central galaxy, due to other satellite galaxies, and due to the potential of the halo itself. These tidal interactions can remove dark matter, gas and stars from the galaxy, a process called tidal stripping , and may also perturb its structure. In addition, if the halo contains a hot gas component, any gas associated with the satellite galaxy will experience a drag force due to the relative motion of the two fluids. If the drag force exceeds the restoring force due to the satellite’s own gravity, its gas will be ablated, a process called ram-pressure stripping. These dynamical processes are thought to play an important role in driving galaxy evolution within clusters and groups of galaxies. In particular, they are thought to be partially responsible for the observed environmental dependence of galaxy morphology .
Internal dynamical effects can also reshape galaxies. For example, a galaxy may form in a configuration which becomes unstable at some later time. Large scale instabilities may then redistribute mass and angular momentum within the galaxy, thereby changing its morphology. A well-known and important example is the bar-instability within disk galaxies. a thin disk with too high a surface density is susceptible to a non-axisymmetric instability, which produces a bar-like structure similar to that seen in barred spiral galaxies. These bars may then buckle out of the disk to produce a central ellipsoidal component, a so-called ‘pseudobulge’. Instabilities may also be triggered in otherwise stable galaxies by interactions. Thus, an important question is whether the sizes and morphologies of galaxies were set at formation, or are the result of later dynamical process (‘secular evolution’, as it is termed). Bulges are particularly interesting in this context. They may be a remnant of the first stage of galaxy formation, or, may reflect an early merger which has grown a new disk, or may result from buckling of a bar. It is likely that all these processes are important for at least some bulges.
