Evolution of Galaxies in the Hubble Sequence

Abstract

The confrontation of theoretical arguments and observational facts leads to view the Hubble sequence as an aging sequence from late to early type galaxies. To illustrate this, a few N-body simulations of disk galaxies are described showing a series of dynamical events such as bulge building must occur in the life of a single isolated galaxy over time-scales much shorter than the Hubble time. We concentrate here on the basic physics ruling galaxies, collisionless gravitational dynamics, because any major of these global morphology modifications must trigger further important consequences in the subsidiary physics associated with gas and star formation. A new kind of instability in 3D models is reported, analogous to the vertical 2/1 instability leading to the formation of peanut-shaped bars: a 1/1 vertical instability in which a secondary bar embedded in a larger one spontaneously inclines its rotation axis and warps the surrounding disk. The role of an outer disk is particularly important to maintain the spiral evolution over several Gyr by allowing a prolonged exchange of angular momentum with the inner disk.