more the curve is dominated by the dark matter halo. As already pointed out, for massive spiral galaxies such as those in the GASS sample the shape of the rotation curve is well behaved outside the bulge (Catinella et al. 2006, Verheijen 2001). However in general, as described by Verheijen (2001), the shape of the curve depends on the galaxy mass and type, and in particular the radius at which the dark matter halo starts to dominate the curve is smaller and smaller for less massive galaxies, with dwarfs having a constantly rising curve out to the last measured point. Therefore, an important source of scatter in the TF relation is driven by the shape of the curve, $dV/dr$, which in turn is determined by the baryonic/dark matter mass ratio or the total $M/L$ ratio (as has been highlighted for the B band in Fig. \[histo\]).

$\bullet$ In the model the velocity dispersion is traced by the bulge stars, since the disk is fully rotationally supported, and therefore we set our "aperture” as encompassing the radial extension of the bulge. Note that this implies a different radius for each galaxy, and corresponds to the assumption that the aperture is always 