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 li ne of neutral hydrogen, is considered in the context of Milgrom's modi fied dynamics (MOND). Combined with the previous, highly selected samp le of Begeman et al., this constitutes the current total sample of gal axies with published (or available) extended rotation curves and photo metric observations of the light distribution. This is the observation al basis of present quantitative understanding of the discrepancy betw een the visible mass and classical dynamical mass in galaxies. It is f ound that the gravitational force calculated from the observed distrib ution of luminous material and gas by use of the simple MOND formula c an account for the overall shape and amplitude of these 22 rotation cu rves, 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 lum inous disk; in nine cases, there is an additional free parameter, whic h is MIL of a central bulge or light concentration. The values of the global MIL (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 rota tion curves in this larger, less stringently selected sample lends fur ther 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.