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.
