Probing for dark matter within spiral galaxy disks

We explore the relative importance of the stellar mass density as compared to the inner dark halo using the observed gas kinematics throughout the dis k of the spiral galaxy NGC 4254 (M99). We perform hydrodynamic simulations of the gas flow for a sequence of gravitational potentials in which we vary the stellar disk contribution to the total potential. This stellar portion of the potential was derived empirically from color-corrected K-band photo metry reflecting the spiral arms in the stellar mass, while the halo was mo deled as an isothermal sphere. The simulated gas density and the gas veloci ty field are then compared to the observed stellar spiral arm morphology an d to the Ha gas kinematics. We find that this method is a powerful tool to determine the corotation radius of the spiral pattern and that it can be us ed to place an upper limit on the mass of the stellar disk. For the case of the galaxy NGC 4254 we find R-CR = 7.5 +/- kpc, or R-CR = 2.1 R-exp (K'). We also demonstrate that for a maximal disk the prominent spiral arms of th e stellar component overpredict the noncircular gas motions unless an axisy mmetric dark halo component contributes significantly (greater than or simi lar to 1/3) to the total potential inside 2.2 K-band exponential disk scale lengths.