Breaking the disk/halo degeneracy with gravitational lensing

The degeneracy between the disk and the dark matter contribution to galaxy rotation curves remains an important uncertainty in our understanding of disk galaxies. Here we discuss a new method for breaking this degeneracy using gravitational lensing by spiral galaxies, and apply this method to the spiral lens B1600+434 as an example. The combined image and lens photometry constraints allow models for B1600+434 with either a nearly singular dark matter halo or a halo with a sizable core. If the dark halo has a core, then the bulge dominates the gravitational potential in the inner part of the galaxy, its mass is between 1.3 and 1.5 x 10(11) M., and the disk mass is less then 5 x 10(10) M.. If the dark halo is singular, there is a degeneracy between the disk mass and the halo ellipticity. The dark halo flattening (c/a) can be as low as 0.53 if there is no disk mass, while the maximum allowed disk mass, 1.3 x 10(11) M., is reached with a spherical halo. A maximum disk model is ruled out with high confidence. Further information, such as the circular velocity of this galaxy, will help break the degeneracies. Future studies of spiral galaxy lenses will be able to determine the relative contribution of disk, bulge, and halo to the mass in the inner parts of galaxies.