The Distribution of Mass in Spiral Galaxies.

Little is known about the content and distribution of dark matter in spiral galaxies. Mass modeling of any rotation curve can yield an alarming range of results - from entirely halo-dominated, centrally- concentrated dark distributions, to disk-dominated inner potentials with shallow, low density halos. To break this degeneracy, we propose to take advantage the the unique properties of the new SparsePak Integral Field Unit on WIYN. To do this successfully, a two-phased observational approach is required, as our pilot study has proven. First, we will obtain H(alpha) velocity fields for a sample of ~100 photometrically selected spiral galaxies to determine the regularity of the kinematics and the inclinations of the disks. These data will also be used to study disk ellipticities and kinematic asymmetries in the context of the scatter in the Tully-Fisher relation. Second, we will measure the stellar kinematics and velocity dispersions for a subsample of about 40 galaxies that were found to have kinematically well behaved disks and suitable inclinations. The stellar velocity dispersions provide a kinematic measurement of the stellar disk mass, thus breaking the degeneracy, making possible a measurement of the dark matter properties in spiral galaxies with unprecedented accuracy