Tully-Fisher relationships

The second is using the Tully-Fisher relationship, which provides a direct relationship between the absolute luminosity of a spiral galaxy and its maximum rotation speed. Since the maximum rotation speed can be directly estimated by observation, one can estimate the absolute luminosity since we measure directly the apparent luminosity, the inverse-square law allows us to estimate the distance to the object concerned. For the present discussion, it is important to understand that, at optical wavelengths, estimates of maximum rotation speeds are generally extrapolations from rotation curve measurements. 

For this sample, the galactic distance scale has been set by Mathewson, Ford, and Buchhorn (MFB) using the Tully-Fisher relationship, which sets distance scales by using an observed correlation between the maximum rotation velocity of a spiral and its absolute luminosity, and so is quite distinct from Hubble-based distance determinations. Even so, the Tully-Fisher method gives an absolute scale only after calibration, and the MFB calibration gave a scale that was statistically similar to a Hubble scale using H = 85kms-1 Mpc 1. 

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