The properties of spiral galaxies: confronting hierarchical galaxy formation models with observations

We compare the properties of local spiral galaxies with the predictions of the Cole et al. semi-analytic model of hierarchical galaxy formation, in order to gain insight into the baryonic processes, such as gas cooling and star formation, that were responsible for shaping these galaxies. On the whole, the models reproduce the properties of presentday spirals rather well, including the trend in scale-size with luminosity, the width of the scale-size distribution, the tight gas fraction–surface brightness and gas fraction– star formation history correlations, the metallicity–magnitude correlation, and the present-day star formation rates and stellar mass-to-light ratios. Of special note is our demonstration that, once the effects of dust and variations in stellar populations have been taken into account, published spiral galaxy scale-size distributions derived from optical data (with widths σ ∼ 0.3) can be reconciled with the width (σ ∼ 0.5) of the stellar mass scale-size distribution predicted by the semi-analytic model. There are some illuminating discrepancies between the observations and the Cole et al. model predictions. The model colours of luminous spiral galaxies are somewhat too blue and those of faint galaxies somewhat too red, most likely indicating shortcomings in the way that gas is accreted by spiral galaxies. Furthermore, the model produces too few luminous spiral galaxies. These difficulties could be resolved by altering the way in which gas cooling is treated or, perhaps, by adopting a higher baryon fraction and invoking galactic “superwinds.” Secondly, stellar mass-to-light ratios are found to be as high as observations permit. Yet, typically 60 per cent of the mass in the inner half-light radius of the model galaxies is dark. This results in an offset between the model and observed spiral galaxy luminosity-linewidth relation. This could be resolved by substantially reducing the mass of baryons which make it into a galaxy disc (with an attendant decrease in stellar mass-to-light ratio), or by modifying the assumed dark matter profile to include less dark matter in the inner parts.