The origin of the Hubble sequence in CDM cosmology

The GALFORM semi-analytic model of galaxy formation is used to explore the mechanisms primarily responsible for the three types of galaxies seen in the local universe: bulge, bulge+disc and disc, identified with the visual morphological types E, S0/a-Sbc and Sc-Scd, respectively. With a suitable choice of parameters the GALFORM model can accurately reproduce the observed local Ks-band luminosity function (LF) for galaxies split by visual morphological type. The successful set of model parameters is used to populate the Millennium Simulation with 9.4 million galaxies and their dark matter haloes. The resulting catalogue is then used to explore the evolution of galaxies through cosmic history. The model predictions concur with recent observational results including the galaxy merger rate, the star formation rate and the seemingly antihierarchical evolution of ellipticals. However, the model also predicts significant evolution of the elliptical galaxy LF that is not observed. The discrepancy raises the possibility that samples of z ∼ 1 galaxies which have been selected using colour and morphological criteria may be contaminated with galaxies that are not actually ellipticals.