Jim Pizagno (Stony Brook) gave a great and very lively group-meeting talk about his work on the Tully–Fisher relation—the narrow power-law relationship between galaxy stellar mass or luminosity and disk rotation velocity—and the scatter about the relation. He did the very striaghtforward thing of attempting to measure rotation curves for a complete sample of galaxies, not selected by subjective morphology. After subtracting out estimates of observational uncertainties in quadrature, he gets a scatter of about 0.4~mag, which is small by any theoretical comparison (and the theoretically predicted scatters don't include scatters in dust content). His talk brought up a lot of side issues too, relating to the metallicities and star-formation histories of the galaxy disks.
After his talk, Pizagno and I discussed his project to make stellar-mass images of SDSS galaxies and re-analyze his spectra in the context of these two-dimensional maps. Unfortunately, the ball is in my court!