HI linewidth will yield a value of the circular velocity consistent with that which would be measured from the rotation curve itself.

$\bullet$ A source of scatter in the Tully-Fisher, whether theoretical or observational, comes from the choice of a radius at which to measure it, or the choice of an aperture within which to collect the signal and determine a linewidth. As described in Yegorova et al. (2007) and Salucci et al. (2007), rotation curves are actually rarely really flat, and measurements encompassing different radii will give rise to different slopes of the TF relation. This is due to the different density profiles of the galaxy components (disk, bulge, dark matter halo), each of which dominate the curve at different radii. As different radii map different dynamical regimes in the galaxy, so the signal in the HI linewidth, or the amplitude of the rotation curve, will be dominated by different components depending on aperture or radius. In the model we choose a radius large enough to be well away from the bulge, and moreover apply our correction of $R_D$ in Eq. (\[rdafter\]) that ’resets’ the curve in the presence of the bulge. However the farther out we go, the 