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 hierar chical clustering scheme of galaxy formation proposed by Cole et al., we assume that galaxies form stars quiescently in a disc until they ar e disrupted into a spheroidal configuration by mergers. Bulges and sph eroids may continue to accrete gas from their hot coronae, and so they may grow discs again. Thus an individual galaxy may pass through vari ous phases of disc or spheroid dominance during its lifetime. To disti nguish between discs and spheroids we add one additional free paramete r to the semianalytic model of Cole et al., which we fix by requiring that the predicted morphological mix should match that observed locall y. Assuming an Ohm = 1, standard cold dark matter cosmology, we calcul ate formation and merging histories, and the evolution in colour, lumi nosity and morphology of the galaxy populations in different environme nts. Our model predicts that the bulges of spirals were assembled befo re the spheroids of ellipticals, and that the spheroids of cluster ell ipticals were assembled before those of field ellipticals. About 50 pe r cent of ellipticals, but only about 15 per cent of spirals, have und ergone a major merger during the redshift interval 0.0 less than or eq ual to z less than or equal to 0.5. In spite of their violent formatio n history, elliptical galaxies turn out to have colour-magnitude diagr ams with remarkably small scatter. Apart from a general blueing of the galaxy population with redshift, the colour-magnitude diagrams are re markably similar at redshift z = 0.5 and at the present day. The morph ological mix of galaxies that become rich cluster members at high reds hift is dominated by spiral galaxies, due to the long time-scale for g alaxy mergers compared with the time-scale for cluster assembly at hig h 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 f ield 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 functio n, on the other hand, evolves rapidly with redshift, increasing its ch aracteristic luminosity and becoming steeper at the faint end. These t rends are similar to those recently observed in the Canada-France Reds hift Survey. Our calculations serve to demonstrate that a simple presc ription for the distinction between discs and spheroids that is compat ible with hierarchical clustering goes a long way towards explaining m any of the systematic trends observed in the galaxy population.