11.3 The Hubble Sequence

In Section 6.5, the relation between spiral structure and the DM fraction in disk galaxies was discussed. It appears that early type galaxies have more centrally concentrated halos, whereas late-type, low-luminosity disk galaxies have such dominant halos that spiral structure is unable to form. These results give some support to the notion that the properties of dark halos have a significant effect on the type of galaxy that eventually forms.

A related suggestion is that barred spirals can only form within fairly insubstantial halos. As noted in Section 9, massive spherical halos suppress bar instabilities. While other mechanisms can suppress bars, it seems that bars form more easily if the disk-to-halo mass ratio inside the optical radius is large. A detailed review of ideas relating to galaxy formation and the Hubble sequence is provided by Silk and Wyse (1992).