We investigate the origins of galaxy morphology ( defined by bulge-to-total K-band luminosity ) in the \Lambda CDM cosmology using two galaxy formation models based on the Millennium simulation , one by Bower et al .
( the Durham model ) and the other by De Lucia & Blaizot ( the MPA model ) .
Both models have had considerable success in reproducing a number of observed properties of the local and high redshift universe , including star formation rates , the stellar mass function and the luminosity function out to z \sim 5 .
There are many similarities , but also fundamental disagreements in the predictions of the two models for galaxy morphology .
For example , taking into account uncertainties in the available observational data , both produce a realistic morphological mix today , but its evolution is very different .
A main cause of this and other differences is the treatment of disk instabilities which play a more prominent role in the Durham model .
Our analysis confirms previous theoretical predictions that elliptical galaxies form most of their stars before the bulk of the galaxy is assembled .
Spirals tend to have later ‘ assembly ’ times than ellipticals as a consequence of in-situ star formation .
With the exception of the brightest ellipticals ( stellar mass M _ { * } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } % } } 2.5 \times 10 ^ { 11 } h ^ { -1 } M _ { \odot } ) , we find that major mergers are not the primary mechanism by which most spheroids ( ellipticals and spiral bulges ) assemble their mass .
In fact , the majority of ellipticals ( and the overwhelming majority of spirals ) never experience a major merger ( above the resolution limit of our simulation . )
Most ellipticals and spiral bulges acquire their stellar mass through minor mergers or disk instabilities .
These conclusions are common to both the MPA and Durham models .
The rotation properties of spheroids may help to constrain the importance of disk instabilities in these models .