Accurate determination of the mass distribution in spiral galaxies. II. Testing the shape of dark halos

New high-resolution Fabry-Perot data at the Canada-France-Hawaii Telescope combined with published VLA 21 cm observations are used to determine the mass distribution of NGC 3109 and IC 2574. The multiwavelength rotation curves allow us to test with confidence different dark halo functional forms from the pseudoisothermal sphere to some popular halo distributions motivated by cold dark matter N-body simulations. It appears that the density distributions with high central concentration, predicted by these simulations, are very hard to reconcile with rotation curves of late-type spiral galaxies. Modified Newtonian dynamics (MOND) is also considered a potential solution to missing mass and is tested the same way. Using the higher resolution Hα data and new H I data for NGC 3109, one can see that MOND can reproduce in detail the rotation curves of IC 2574 and NGC 3109. However, the value for the MOND universal constant is ∼2 times larger than the value found for more massive spiral galaxies.