Calibrating the TF relation using Cepheid Distances

The zero-point of the Tully Fisher relation needs to be calibrated using galaxies with accurate known distances - namely, those with measured Cepheid's. This was one of the tasks for the HST Key Project to measure H0. The relations shown here in 4 bands also exhibit the systematic decrease in scatter and slope from blue to red bands. But here, the y-axis -- the luminosities -- are now properly calibrated.