GALAXY FORMATION IN A VARIETY OF HIERARCHICAL-MODELS

We predict the observable properties of the galaxy population in several popular hierarchical models of galaxy formation. We employ a detailed semi-analytic procedure which incorporates the formation and merging of dark matter haloes, the shock heating and radiative cooling of gas, self-regulated star formation, the merging of galaxies within dark matter haloes, and the spectral evolution of the stellar populations. We contrast the standard CDM cosmogony with variants of the CDM models having either a low value of H-0, or a low value of Omega with or without a cosmological constant. In addition, we compare galaxy formation in these CDM universes with a 'cold plus hot' dark matter model (CHDM). The cosmological parameters in these models are constrained by observations of large-scale structure, light-element abundances, and globular cluster ages, while the astrophysical parameters we determine by attempting to produce the best fit to the present-day observed B-band luminosity function. Having fixed the parameters in this manner we gauge the success or failure of each model by comparison with other properties of the observed galaxy population: the K-band luminosity function, the infrared Tully-Fisher relation, B - K colours, number counts and redshift distributions. We find that, although the models have some success in remedying the shortcomings of the standard CDM cosmogony, none of these new models produces broad agreement with the whole range of observations. Although the low-Omega and Omega + h = 1 CDM models reduce the discrepancy between the predicted and observed Tully-Fisher relations (the main weakness of galaxy formation in standard CDM), these models predict an inverted colour-magnitude relation and do not produce an exponential cut-off at the bright end of the galaxy luminosity function. All of our models predict recent star formation in the majority of galaxies and exhibit galaxy colours bluer than observed, but this problem is far more severe in the CHDM model, which produces colours about two magnitudes too blue in B - K. The excessively blue colours in the CHDM model are a direct consequence of the late epoch of structure formation and are independent of our modelling of star formation and feedback. We discuss several potential refinements to the galaxy formation recipe: the inclusion of metallicity effects, non-local feedback, inhibited star formation in cooling flows and an initial mass function that varies in time and space.