The Interstellar Medium of Lens Galaxies

We use observations from the CASTLES survey of gravitational lenses to study extinction in 23 lens galaxies with $0<z_l<1$. The median differential extinction between lensed images is $\mathrm{\Delta }E(\text{bv})=0.05$ mag, and the directly measured extinctions agree with the amount needed to explain the differences between the statistics of radio and (optical) quasar lens surveys. We also measure the first extinction laws outside the local universe, including an $R_V=7.2$ curve for a molecular cloud at $z_l=0.68$ and an $R_V=1.5$ curve for the dust in a redshift $z_l=0.96$ elliptical galaxy.