The Published Extended Rotation Curves of Spiral Galaxies: Confrontation with Modified Dynamics

A sample of 22 spiral galaxy rotation curves, measured in the 21 cm line of neutral hydrogen, is considered in the context of Milgrom's modified dynamics (MOND). Combined with the previous, highly selected sample of Begeman et al., this constitutes the current total sample of galaxies with published (or available) extended rotation curves and photometric observations of the light distribution. This is the observational basis of present quantitative understanding of the discrepancy between the visible mass and classical dynamical mass in galaxies. It is found that the gravitational force calculated from the observed distribution of luminous material and gas by use of the simple MOND formula can account for the overall shape and amplitude of these 22 rotation curves, and in some cases, the predicted curve agrees with the observed rotation curve in detail. The fitted rotation curves have, in 13 cases, only one free parameter, which is the mass-to-light ratio of the luminous disk; in nine cases, there is an additional free parameter, which is M/L of a central bulge or light concentration. The values of the global M/L (bulge plus disk) are reasonable and, when the gas mass is also included, show a scatter consistent with that in the Tully-Fisher relation. The success of the MOND prescription in predicting the rotation curves in this larger, less stringently selected sample lends further support to the idea that dynamics or gravity is non-Newtonian in the limit of low acceleration and that it is unnecessary to invoke the presence of large quantities of unseen matter.