Evolution of the Hubble sequence in hierarchical models for galaxy formation

We present a model for the broad morphological distinction between the disc and spheroidal components of galaxies. Elaborating on the hierarchical clustering scheme of galaxy formation proposed by Cole et al., we assume that galaxies form stars quiescently in a disc until they are disrupted into a spheroidal configuration by mergers. Bulges and spheroids may continue to accrete gas from their hot coronae, and so they may grow discs again. Thus an individual galaxy may pass through various phases of disc or spheroid dominance during its lifetime. To distinguish between discs and spheroids we add one additional free parameter to the semianalytic model of Cole et al., which we fix by requiring that the predicted morphological mix should match that observed locally. Assuming an Ohm = 1, standard cold dark matter cosmology, we calculate formation and merging histories, and the evolution in colour, luminosity and morphology of the galaxy populations in different environments. Our model predicts that the bulges of spirals were assembled before the spheroids of ellipticals, and that the spheroids of cluster ellipticals were assembled before those of field ellipticals. About 50 per cent of ellipticals, but only about 15 per cent of spirals, have undergone a major merger during the redshift interval 0.0 less than or equal to z less than or equal to 0.5. In spite of their violent formation history, elliptical galaxies turn out to have colour-magnitude diagrams with remarkably small scatter. Apart from a general blueing of the galaxy population with redshift, the colour-magnitude diagrams are remarkably similar at redshift z = 0.5 and at the present day. The morphological mix of galaxies that become rich cluster members at high redshift is dominated by spiral galaxies, due to the long time-scale for galaxy mergers compared with the time-scale for cluster assembly at high redshift. The assembly of low-redshift clusters is slower, allowing more galaxy mergers to occur in the progenitor haloes. As a result, z = 0 rich clusters become E/S0 dominated, and we find a 'Butcher-Oemler' effect that becomes weaker for poorer groups at high redshift. The field luminosity function of red galaxies shows little evolution out to z similar or equal to 1, and the reddest galaxies at these redshifts are as bright as their local counterparts. The blue luminosity function, on the other hand, evolves rapidly with redshift, increasing its characteristic luminosity and becoming steeper at the faint end. These trends are similar to those recently observed in the Canada-France Redshift Survey. Our calculations serve to demonstrate that a simple prescription for the distinction between discs and spheroids that is compatible with hierarchical clustering goes a long way towards explaining many of the systematic trends observed in the galaxy population.