Galaxy Mass Models: MOND versus Dark Matter Halos

Mass models of 15 nearby dwarf and spiral galaxies are presented. The galaxies are selected to be homogeneous in terms of the method used to determine their distances, the sampling of their rotation curves (RCs) and the mass-to-light ratio (M/L) of their stellar contributions, which will minimize the uncertainties on the mass model results. Those RCs are modeled using the MOdified Newtonian Dynamics (MOND) prescription and the observationally motivated pseudo-isothermal (ISO) dark matter (DM) halo density distribution. For the MOND models with fixed (M/L), better fits are obtained when the constant a$_{0}$ is allowed to vary, giving a mean value of (1.13 $\pm$ 0.50) $\times$ 10$^{-8}$ cm s$^{-2}$, compared to the standard value of 1.21 $\times$ 10$^{-8}$ cm s$^{-2}$. Even with a$_{0}$ as a free parameter, MOND provides acceptable fits (reduced $\chi^{2}_{r}$ $<$ 2) for only 60% (9/15) of the sample. The data suggest that galaxies with higher central surface brightnesses tend to favor higher values of the constant a$_{0}$. This poses a serious challenge to MOND since a$_{0}$ should be a universal constant. For the DM models, our results confirm that the DM halo surface density of ISO models is nearly constant at $\rho_{0} \ R_{C} \sim 120\ M_{\odot} \ pc^{-2}$. This means that if the (M/L) is determined by stellar population models, ISO DM models are left with only one free parameter, the DM halo central surface density.