Bias properties of extragalactic distance indicators. VII. Correlation of absolute luminosity and rotational velocity for Sc galaxies over the range of luminosity class from I to III-IV

A distance-limited subset of the complete flux-limited sample of Sc galaxie s in the Revised Shapley-Ames Catalog of Bright Galaxies is isolated by mea ns of separate Spaenhauer diagrams for six individual van den Bergh luminos ity class intervals from Sc I + I.2,.3 to Sc III-IV. The distribution funct ions of kinematic absolute B-T(0, i)(220, 50) magnitudes and 21 cm line wid ths, W-20, corrected to edge-on orientation, have been determined for the s ame six bins of luminosity class. The individual luminosity functions for e ach luminosity class are bounded on both the bright and faint ends, showing that the present sample includes no dwarf Sc spirals fainter than M(B-T)(2 20, 50) = -18 belonging to luminosity classes I to III-IV, as defined by th e regularity of the spiral pattern. Star-forming galaxies with spiral struc tures as regular as the ones found in these luminosity classes have absolut e magnitudes brighter than M-B(H = 50) = -18 and 21 cm line widths larger t han W-20/sin i = 2 upsilon(rot)(max) = 165 km s(-1). Furthermore, the 21 cm line-width distributions move toward smaller rotational velocities as the luminosity classes change from I to III, showing that rotation is a princip al parameter determining the regularity of the spiral pattern. Whether it i s the only parameter awaits a similar investigation for spirals of all lumi nosity classes along the complete Hubble sequence. In particular, it has no t yet been proved that all Im and Sm galaxies, where, by definition, the sp iral arms are either lacking or are semichaotic, have absolute magnitudes t hat are fainter than M-B = -18 and whose 21 cm LWs are smaller than similar to 165 km s(-1), presumably because of smaller mass than the high-luminosi ty, regular spirals. The Teerikorpi "cluster population incompleteness bias " is demonstrated again. Here, however, as in Papers II-IV of this series, we use field galaxies to show that the slope and zero point of the Tully-Fi sher (T-F) relation are systematically incorrect for flux-limited samples, the error increasing with redshift. The data also support the possibility t hat the zero point of the T-F relation for Sc galaxies, even in the distanc e-limited sample, is a function of luminosity class, varying by 0.42 mag be tween Sc I + I.2,.3 and Sc III galaxies if the slope of the T-F relation fo r each class is taken to be that of the fundamental calibrating galaxies wi th Cepheid distances. If true, this is presumably due to differences in the mass distribution and, therefore, in the resulting rotational velocity fie ld as a function of luminosity class even within a fixed Hubble type.