Evolution of the Hubble sequence in hierarchical models for galaxy formation

ABSTRACT We present a model for the broad morphological distinction between the disk andspheroidal components of galaxies. Elaborating on the hierarchical clustering scheme ofgalaxy formation proposed by Cole et al, we assume that galaxies form stars quiescentlyin a disk until they are disrupted into a spheroidal configuration by mergers. Bulges andspheroids may continue to accrete gas from their hot coronae, and so they may growdisks again. Thus, an individual galaxy may pass through various phases of disk orspheroid dominance during its lifetime. To distinguish between disks and spheroids weadd one additional free parameter to the semianalytic model of Cole et al. which we fixby requiring that the predicted morphological mix should match that observed locally.Assuming an Ω = 1, standard cold dark matter cosmology, we calculate formation andmerging histories, and the evolution in colour, luminosity and morphology of the galaxypopulations in different environments.Our model predicts that the bulges of spiralswereassembled before the spheroids of ellipticals and the spheroids of cluster ellipticals wereassembled before those of field ellipticals. About 50% of ellipticals, but only about 15%of spirals, have undergone a major merger during the redshift interval 0.0 ≤ z ≤ 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 ofthe galaxy population with redshift, the colour-magnitude diagrams are remarkablysimilar at redshift z = 0.5 and at the present day. The morphological mix of galaxiesthat become rich cluster members at high redshift is dominated by spiral galaxies,due to the long timescale for galaxy mergers compared with the timescale for clusterassembly at high redshift. The assembly of low redshift clusters is slower, allowingmore galaxy mergers to occur in the progenitor halos. As a result z = 0 rich clustersbecome E/S0 dominated and we find a “Butcher-Oemler” effect that becomes weakerfor poorer groups at high redshift. The field luminosity function of red galaxies showslittle evolution out to z ≃ 1 and the reddest galaxies at these redshifts are as brightas their local counterparts. The blue luminosity function, on the other hand, evolvesrapidly with redshift, increasing its characteristic luminosity and becoming steeper atthe faint end. These trends are similar to those recently observed in the Canada-FranceRedshift Survey.Our calculationsserveto demonstratethat a simple prescriptionfor thedistinction between disks and spheroids that is compatible with hierarchical clusteringgoes a long way towards explaining many of the systematic trends observed in thegalaxy population.Key words: Galaxies:evolution-galaxies: formation-galaxies.