Measuring Stellar and Dark Mass Fractions in Spiral Galaxies

Abstract

We explore the relative importance of the stellar mass density as compared to the inner dark halo, for the observed gas kinematics thoughout the disks of spiral galaxies. We perform hydrodynamical simulations of the gas flow in a sequence of potentials with varying the stellar contribution to the total potential. The stellar portion of the potential was derived empirically from K-band photometry. The output of the simulations — namely the gas density and the gas velocity field — are then compared to the observed spiral arm morphology and the Hα gas kinematics. We solve for the best matching spiral pattern speed and draw conclusions on how massive the stellar disk can be at most. For the case of the galaxy NGC 4254 (Messier 99) we demonstrate that the prominent spiral arms of the stellar component would overpredict the non-circular gas motions unless an axisymmetric dark halo component adds significantly in the radial range R^exp<R<3 R^exp.