A TECHNIQUE FOR USING RADIO JETS AS EXTENDED GRAVITATIONAL LENSING PROBES

We propose a new and potentially powerful method of measuring the mass of a galaxy (or dark matter concentration) which lies close in position to a background polarized ratio jet. Using the fact that the polarization angel is not changed by lensing, we define an "alignment-breaking parameter" eta-G, which we show to be a sensitive indicator of gravitational distortion. The method remains sensitive over a wide redshift range of the gravitational lens. We apply our technique to the analysis of polarimetric observations of the jet of 3C 9 at z = 2.012 combined with a newly discovered 20.3 mag foreground galaxy at z = 0.2538 to "weigh" the galaxy and obtain an approximate upper limit to the mass-to-light ratio.