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 present-day 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 \sigma \sim 0.3 ) can be reconciled with the width ( \sigma \sim 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 .