Extended rotation curves of high-luminosity spiral galaxies. V. NGC 1961, the most massive spiral known.

The rotation curve derived for NGC 1961 indicates a total mass greater than 10 to the 12th solar masses. This large value is confirmed by the correspondingly large mass, more than 10 to the 11th solar masses, for the interstellar H I component derived from 21-cm measurements. This supergiant spiral has normal ratios formed from values of its total mass, H I content, and blue luminosity. The optically derived spectra indicate peculiar and unexplained motions within the system.