A RECIPE FOR GALAXY FORMATION

We present a detailed prescription for the modelling of galaxy formati on in hierarchical theories of structure formation. Our model incorpor ates the formation and merging of dark matter haloes, the shock heatin g and radiative cooling of baryonic gas gravitationally confined in th ese haloes, star formation regulated by the energy released by evolvin g stars and supernovae, the merging of galaxies within dark matter hal oes, and the spectral evolution of the stellar populations that are fo rmed. The procedure that we describe is very flexible and can be appli ed to any hierarchical clustering theory. Our prescriptions for regula ted star formation and galaxy mergers are motivated and constrained by numerical simulations. We are able to predict galaxy numbers, luminos ities, colours and circular velocities. This investigation is restrict ed to the standard cold dark matter (CDM) cosmology, and we explore th e effects of varying other assumptions, including the stellar initial mass function, star formation rates and galaxy merging. We compare the results of these models with an extensive range of observational data , including the B and K galaxy luminosity functions, galaxy colours, t he Tully-Fisher relation, faint galaxy number counts, and the redshift distribution at B approximate to 22. This combination of observed gal axy properties strongly constrains the models and enables the relative importance of each of the physical processes included to be assessed. We present a broadly successful model defined by a plausible choice o f parameters. This fiducial model produces a much more acceptable lumi nosity function than have most previous studies. This is achieved thro ugh a modest rate of galaxy mergers and strong suppression of star for mation in haloes of low circular velocity by energy injected by supern ovae and evolving stars. The model also accounts for the observed fain t galaxy counts in both the B and K bands, and their redshift distribu tions. It fails, however, to produce galaxies as red as are many obser ved ellipticals and, compared with the observed Tully-Fisher relation, the model galaxies have circular velocities which are too large for t heir luminosities.