The first precise determination of an optical-far-ultraviolet extinction curve beyond the local group (z=0.83)

We present the optical-far-ultraviolet extinction curve of the dust in the lens galaxy of the gravitational lens system SBS 0909+ 532 (z = 0.83). Extending our previous optical-UV estimate (from lambda(0)(-1) similar to2 to 5 mum(-1)) into the far ultraviolet (from lambda(0)(-1) similar to 5 to 8 mum(-1)) is crucial for comparing with the extinction curves measured for Local Group galaxies in the spectral region where the differences are greatest. The SBS 0909+ 532 curve is similar to that of the LMC2 supershell, with a weaker 2175 Angstrom feature and a steeper rise into the UV than that observed in the Milky Way. The shapes of the extinction curve inferred from the quasar continuum and emission lines are in very good agreement. There is, however, a 0.6 +/- 0.1 mag offset in the implied magnification of the source that can be interpreted as differential magnification due to microlensing of the compact accretion disk producing the continuum emission by the stars in the lens galaxy. There is no evidence for a wavelength dependence on the microlensing effect except for a similar to2 sigma shift near the Mg II emission line. If this difference were confirmed, it could be used to probe the emission profile of the quasar accretion disk.