A Study of Gravitational Lens Chromaticity with the Hubble Space Telescope

We report Hubble Space Telescope observations of 6 gravitational lenses with the Advanced Camera for Surveys. We measured the flux ratios between the lensed images in 6 filters from 8140Å to 2200Å. In 3 of the systems, HE0512$-$3329, B1600+434, and H1413+117, we were able to construct UV extinction curves partially overlapping the 2175Å feature and characterize the properties of the dust relative to the Galaxy and the Magellanic Clouds. In HE1104$-$1804 we detect chromatic microlensing and use it to study the physical properties of the quasar accretion disk. For a Gaussian model of the disk $\exp(-r^2/2 r_s^2)$, scaling with wavelength as $r_s \propto λ^p$, we estimate $r_s(\lambda3363)=4^{+4}_{-2}$ ($7\pm 4$) light-days and $p=1.1\pm 0.6$ ($1.0\pm 0.6$) for a logarithmic (linear) prior on $r_s$. The remaining two systems, FBQ0951+2635 and SBS1520+530, yielded no useful estimates of extinction or chromatic microlensing.