therefore their $M/L$ is higher (for instance in the K band, the very luminous post-main sequence phases have faded away; Maraston 2005), while at the same time the velocity is higher due to the boost the bulge inflicts on the rotation curve, and 2) the scatter in the bulge-to-disk mass ratio increases with mass, because larger galaxies have a wider variety of merger histories. Therefore an Sa galaxy that by chance has a smaller than average bulge will scatter back towards the main TF relation, while an Sa galaxy with a larger than average bulge will scatter further away from the TF, and more so with increasind galaxy mass. The overall effect is that of flattening the TF slope. In addition, at the very high-mass end of the mass distribution AGN feedback causes a further increase of the $M/L$ ratio.

Model and data roughly occupy the same locus in the plot (ignoring the data outliers for the moment), so we use the model to provide insight into the observed galaxies.

In the model, the galaxy type and its evolutionary history (short $vs$ prolongued star formation history, merger-rich $vs$ merger-poor assembly) is at first order characterised by the mass ratio between 