A recipe for galaxy formation

We present a detailed prescription for how galaxy formation can be modelledin hierarchical theories of structure formation. Our model incorporates theformation and merging of dark matter halos, the shock heating and radiativecooling of baryonic gas gravitationally confined in these halos, the formationof stars regulated by the energy released by evolving stars and supernovae, themerging of galaxies within dark matter halos, and the spectral evolution of thestellar populations that are formed. The procedure that we describe is veryflexible and can be applied to any hierarchical clustering theory. We explorethe effects of varying the stellar initial mass function, star formation ratesand galaxy merging. The results we compare with an extensive range ofobservational data, including the B and K galaxy luminosity functions, galaxycolours, the Tully-Fisher relation and galaxy number counts.These data stronglyconstrain the models and enable the relative importance of each of the physicalprocess to be assessed. We present a broadly successful model defined by aplausible choice of parameters. This fiducial model produces a much moreacceptable luminosity function than most previous studies. This is achievedthrough a modest rate of galaxy mergers and strong suppression of starformation in halos of low circular velocity. However, it fails to producegalaxies as red as many observed ellipticals and, compared with the observedTully-Fisher relation, the model galaxies have circular velocities which aretoo large. ** uuencoded compressed postscript file containing all text andfigures.*