15.2. Evolution along the Hubble Sequence
We have already shown how non-axisymmetric pertubations, and in particular bars, can provoke rapid evolution of spiral disks, through angular momentum transfers. The presence of the dissipative gas component plays a fundamental role in this evolution. Tidal interactions by companions and mergers also contribute to a rapid evolution. Therefore, galaxies can change Hubble types frequently during their life-time, and the most likely sense of evolution is from late to early types (Pfenniger 1993). What are the consequences for secondary structures like rings and lenses?
The very existence of true detached outer rings in early-type galaxies already tells us that the evolution slows down at the end of the Hubble sequence. The formation time of these structures is a few Gyr, and the bar and its pattern speed must have been steady over this period of time. This is easy to understand since the gas fraction is very low in these early-types, and the gas component is the motor of the evolution. Early-type barred galaxies with massive bulges or high mass concentration should be in the process of slow bar dissolution, and should be accompanied by a lens. This appears to be the case since ~ 54% of SB0-a galaxies have lenses (Kormendy 1979a). Non-barred galaxies with rings can be interpreted as systems where the bar has been slowly dissolved, and axi-symmetric rings have survived, possibly maintained by the remaining oval distortion.