Detection of the 2175 A Extinction Feature atz= 0.83

We determine the extinction curve in the z_l=0.83 lens galaxy of thegravitational lens SBS0909+532 from the wavelength dependence of the flux ratiobetween the lensed quasar images (z_s=1.38) from 3400 to 9200\AA. It is thefirst measurement of an extinction curve at a cosmological distance ofcomparable quality to those obtained within the Galaxy. The extinction curvehas a strong 2175\AA feature, a noteworthy fact because it has been weak ornon-existent in most estimates of extinction curves outside the Galaxy. Theextinction curve is fitted well by a standard $R_V=2.1\pm0.9$ Galacticextinction curve. If we assume standard Galactic extinction laws, the estimateddust redshift of $z=0.88\pm0.02$ is in good agreement with the spectroscopicredshift of the lens galaxy. The widespread assumption that SMC extinctioncurves are more appropriate models for cosmological dust may be incorrect.Comment: 17 pages, 5 figures. ApJ Accepted (Apr 4