Mass and Dust in the Disk of a Spiral Lens Galaxy

Gravitational lensing is a potentially important probe of spiral galaxy structure, but only a few cases of lensing by spiral galaxies are known. We present Hubble Space Telescope and Magellan observations of the two-image quasar PMN J2004–1349, revealing that the lens galaxy is a spiral galaxy. One of the quasar images passes through a spiral arm of the galaxy and suffers 3 magnitudes of V -band extinction. Using simple lens models, we show that the mass quadrupole is well-aligned with the observed galaxy disk. A more detailed model with components representing the bulge and disk gives a bulge-to-disk mass ratio of 0.16 ± 0.05. The addition of a spherical dark halo, tailored to produce an overall flat rotation curve, does not change this conclusion. Subject headings: galaxies: spiral, structure, halos — dark matter — gravitational lensing — dust, extinction — quasars, individual (PMN J2004–1349)