Dust-obscured star formation and AGN fuelling in hierarchical models of galaxy evolution

A large fraction of the luminous distant submillimetre-wave galaxies recent ly detected using the Submillimetre Common-User Bolometer Array (SCUBA) cam era on the James Clerk Maxwell Telescope appear to be associated with inter acting optical counterparts. We investigate the nature of these systems usi ng a simple hierarchical clustering model of galaxy evolution, in which the large luminosity of the SCUBA galaxies is assumed to be generated at the e poch of galaxy mergers in a burst of either star formation activity or the fuelling of an active galactic nucleus (AGN). The models are well constrain ed by the observed spectrum of the far-infrared/submillimetre-wave backgrou nd radiation and the 60-mu m counts of low-redshift IRAS galaxies. The rati o between the total amount of energy released during mergers and the mass o f dark matter involved must increase sharply with redshift z at z less than or similar to 1, and then decrease at greater redshifts. This result is in dependent of the fraction of the luminosity of mergers that is produced by starbursts and AGN. One additional parameter - the reciprocal of the produc t of the duration of the enhanced luminosity produced by the merger and the fraction of mergers that induce an enhanced luminosity, which we call the activity parameter - is introduced, to allow the relationship between mergi ng dark matter haloes and the observed counts of distant dusty galaxies to be investigated. The observed counts can only be reproduced if the activity parameter is greater by factors of about 5 and 100 at redshifts of 1 and 3 respectively, compared with the present epoch. Hence, if merging galaxies account for the population of SCUBA galaxies, then the merger process must have been much more violent at high redshifts. We discuss the counts of gal axies and the intensity of background radiation in the optical/near-infrare d wavebands in the context of these hierarchical models, and thus investiga te the relationship between the populations of submillimetre-selected and L yman-break galaxies.