Motion of the Galaxy and the local group determined from the velocity anisotropy of distant SC I galaxies. I - The data

For a homogeneous all-sky sample of faint Sc I-Sc II galaxies, we present velocities, magnitudes, and diameters. The 96 galaxies, 3500 <V< 6500 kml sec, are used to evaluate the coefficients describing the change in apparent magnitude with path length through the Galaxy, with path length through each galaxy, with luminosity class, and with linear diameter. The sample is also used to evaluate the coefficients describing the decrease in apparent diameter with path length through the Galaxy and through the atmosphere. Such corrections must be applied to remove all systematic effects as a function of position on the sky before a study of the Hubble flow can be properly undertaken. The results indicate a low value for the galactic extinction, A B = 0.15+0.03, and a smaller dependence of magnitude and diameter on inclination than is generally adopted, A = 0.12+0.08 and D = 0. A major conclusion from the analysis is that a well-defined relation between linear diameter and luminosity makes the linear diameter of a high-luminosity Sc galaxy a valuable luminosity discriminant. Assigning to each galaxy an absolute magnitude dependent upon its linear diameter produces a significant decrease in the scatter in the sample; the dispersion in absolute magnitude is oM o<0" 33 near that of brightest ellipticals in clusters. Hence, high-luminosity Sc galaxies are a useful tool for mapping the velocity field, r < 100 Mpc, where cluster ellipticals are few, and not well distributed on the sky.