  Galaxy bimodality is caused by the bulge-disc nature of galaxies as opposed
to two distinct galaxy classes. This is evident in the colour-structure plane
which clearly shows that elliptical galaxies (bulge-only) lie in the red
compact peak and late-type spiral galaxies (disc-dominated) lie in the blue
diffuse peak. Early-type spirals (bulge plus disc systems) sprawl across both
peaks. However after bulge-disc decomposition the bulges of early-type spirals
lie exclusively in the red compact peak and their discs in the blue diffuse
peak (exceptions exist but are rare, e.g., dust reddened edge-on discs and blue
pseudo-bulges). Movement between these two peaks is not trivial because whilst
switching off star-formation can transform colours from blue to red, modifying
the orbits of ~1 billion stars from a planar diffuse structure to a triaxial
compact structure is problematic (essentially requiring an equal mass merger).
We propose that the most plausible explanation for the dual structure of
galaxies is that galaxy formation proceeds in two stages. First an initial
collapse phase (forming a centrally concentrated core and black hole), followed
by splashback, infall and accretion (forming a planar rotating disc). Dwarf
systems could perhaps follow the same scenario but the lack of low luminosity
bulge-disc systems would imply that the two components must rapidly blend to
form a single flattened spheroidal system.
