Scatter of SC galaxies on the Tully-Fisher diagram and a dark matter problem.

A sample of 130 Sc galaxies with photometric I-band profiles, measured by Freudling (1990), and with reliable data on the width of the H I line is considered. As a distance indicator on the diagram, different global parameters of the galaxies were tested. The use of any combinations of optical and radio parameters of galaxies does not decrease the spread of estimates of galactic distances lower than a minimum value of 0.07-0.06. This limit is not significantly influenced by the galactic environment either. About three-fifths of the galaxies exhibit significant deviations of their brightness profile with respect to a standard linear one. However, an amplitude and a sign of the deviation do not correlate with a galaxy location on the TF diagram. This feature may be related to the presence of a dark matter component in the galaxies. The mean ratio of the total mass to the I-luminosity is nearly the same (about 3 solar masses/solar luminosity) both for giant and dwarf galaxies, but it depends on their surface brightness.