Gravitational Lensing by Spiral Galaxies

We study gravitational lensing by spiral galaxies, using realistic modelsconsisting of halo, disk, and bulge components combined to produce a flatrotation curve. Proper dynamical normalization of the models is criticalbecause a disk requires less mass than a spherical halo to produce the samerotation curve---a face-on Mestel disk has a lensing cross section only 41% aslarge as a singular isothermal sphere with the same rotation curve. The crosssection is sensitive to inclination and dominated by edge-on galaxies, whichproduce lenses with an unobserved 2-image geometry and a smaller number ofstandard 5-image lenses. Unless the disk is unreasonably massive, disk+halomodels averaged over inclination predict \lesssim 10% more lenses than purehalo models. Finally, models with an exponential disk and a central bulge aresensitive to the properties of the bulge. In particular, an exponential diskmodel normalized to our Galaxy cannot produce multiple images without a bulge,and including a bulge reduces the net flattening of edge-on galaxies. Thedependence of the lensing properties on the masses and shapes of the halo,disk, and bulge means that a sample of spiral galaxy lenses would provideuseful constraints on galactic structure.Comment: 27 pages, 7 postscript figures, submitted to Ap