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We examine the sources of scatter in the H-band (1.6 m) Tully-Fisher (TF) relation and conclude that it is significantly larger than can be accounted for by observational errors. Using I-band luminosity profiles, we find that the major source of additional scatter stems from intrinsic variations in disk galaxy surface brightness at fixed line width. Moreover, curvature in the TF relation can result when apertures which are too small are used to measure the light in low line width galaxies. To correct for this behavior we have utilized a scheme where total I magnitudes are used for spirals with line widths less than 230 km s-1 and magnitudes within the I = 22.5 mag arcsec-2 isophote are used for galaxies of larger line width. Adhering to this scheme yields TF relations that have a scatter of 0.3 mag. However, if we use the distance-independent luminosity profile itself as a discriminant, we are able to produce an I-band TF relation which exhibits a scatter of only 0.2 mag. To take maximum advantage of this reduction in scatter, care must be taken to assemble a set of calibrating galaxies whose I-band surface brightness profiles most closely resemble those of H I selected spirals in distant clusters. Without a good match, the TF relation may never provide distances to individual galaxies that are better than 20%. In its practical application we find that the slope of I-band TF relation is quite similar for the Pisces and A2634 cluster samples. The implied difference in distance moduli is 1.07 ± 0.09, in good agreement with the value of 1.06 ± 0.15 derived by Aaronson et al. using the H-band TF relation. In addition, we have used the Pisces relation as a fiducial calibrating sequence to search for a zero point offset between a sample of field galaxies, most with redshifts greater than 3000 km s-1, that are located at the North and South Supergalactic Poles. Such an offset would reflect motion that is directed perpendicular to the supergalactic plane. No significant offset was found at a 1 σ limit of 350 km s-1.
Bothun et al. (Sun,) studied this question.