on the fact that the stellar populations of bulge and disk are *visibly* different, and that might not be the case, especially for more massive and evolved systems. Even the most sophisticated photometric schemes in fact cannot easily disentangle the effects of age, metallicity, and reddening (see Pforr et al. 2012, 2013). The degeneracies in luminosity and colours of different stellar populations confuse the mass-to-light ratio and the decomposition of bulge and disk, and we believe this is one factor at the origin of the increased observed scatter in the TF relation for these galaxy types. In fact, the mass-to-light ratio, by mirroring the balance between different dynamical components, is the main factor that shapes the rotation curve, which in turn determines the slope of the TF relation.

Comparison with observations
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To compare the model with observed data, we employ the sample of massive galaxies of GASS (GALEX Arecibo SDSS) survey, Data Release 3 (Catinella et al. (2013; see also Catinella et al. 2010 for a complete description of the survey), which provides circular velocities obtained from HI linewidths. To this sample we add measurements of the central velocity dispersions measurements from the Sloan Digital Sky Survey, Data Release 