On the Use of Galaxy Rotation Curves to Test the Modified Dynamics

I expound various aspects of the testing of the predictions of the modified Newtonian dynamics (MOND) on galaxy rotation curves. This is done with particular reference to the extensive study of rotation curves, in light of MOND, published recently by Kent. The general properties of MOND rotation curves are discussed. In particular, it is shown how the various galaxy parameters (distance, inclination, stellar M/L, etc.) and the acceleration constant of MOND enter the comparison of calculated with observed curves. This exposition, it is hoped, will help optimize the efficacy of tests of MOND that are based on rotation curves, as it shows what sort of information can be extracted from various types of galaxies. I emphasize the crucial role played by low-surface-density galaxies (somewhat neglected by Kent) in assaying the modified dynamics. Kent points out two possible difficulties for MOND that are brought to light by his analysis. They concern the spread of the values he gets for the acceleration constant, and the behavior of some rotation curves at large radii. Examining Kent's procedure, I find that both these potentially deleterious conclusions may be imputed in part to the omission of the observed H I mass from the mass models he uses. The gas contributes to the mass substantially in some cases (more than half the stellar contribution) and is distributed differently from the light. In one other consequential case (which gave the smallest value for a_0_), Kent uses the two-side-average rotation curve of a galaxy (NGC 3031) that has a highly asymmetrical curve. Some revised results provided by Kent are given, viz., new best-fit values of a_0_ for mass models that include hydrogen, and the MOND rotation curves with a_0_ fixed for all the galaxies. I also comment on the possible significance of secondary features on the rotation curve.