Dust and extinction curves in galaxies with z > 0: The interstellar mediumof gravitational lens galaxies

We determine 37 differential extinctions in 23 gravitational lens galaxies over the range 0 less than or similar to z(l) less than or similar to 1. On ly seven of the 23 systems have spectral differences consistent with no dif ferential extinction. The median differential extinction for the optically selected (radio-selected) subsample is Delta E(B-V)= 0.04 (0.06) mag. The e xtinction is patchy and shows no correlation with impact parameter. The med ian total extinction of the bluest images is E(B - V) = 0.08 mag, although the total extinction distribution is dominated by the uncertainties in the intrinsic colors of quasars. The directly measured extinction distributions are consistent with the mean extinction estimated by comparing the statist ics of quasar and radio lens surveys, thereby confirming the need for extin ction corrections when using the statistics of lensed quasars to estimate t he cosmological model. A disjoint subsample of two face-on, radio-selected spiral lenses shows both high differential and total extinctions, but stand ard dust-to-gas ratios combined with the observed molecular gas column dens ities overpredict the amount of extinction by factors of 2-5. For several s ystems we can estimate the extinction law, ranging from R-v = 1.5 +/- 0.2 f or a z(l) = 0.96 elliptical, to R-v = 7.2 +/- 0.1 for a z(l) = 0.68 spiral. For the four radio lenses where we can construct nonparametric extinction curves, we find no evidence for gray dust over the IR-UV wavelength range. The dust can be used to estimate lens redshifts with reasonable accuracy, a lthough we sometimes find two degenerate redshift solutions.