GALAXY MASS DEDUCED FROM THE STRUCTURE OF EINSTEIN RING MG1654+1346

ELLIPTICAL galaxies acting as gravitational lenses occasionally produce spectacular images - Einstein rings - of distant objects. Giant arcs1 and radio rings2 have been observed. A wide variety of image morphologies is possible, the generation of which is qualitatively understood in terms of large magnifications at caustic and critical lines in the lensing geometry3-5. From the angular size of the image, and with knowledge of the distances of the lensed and lensing object, rough estimates of the mass of the lensing galaxy can be obtained. We have made high-resolution radio-interferometric observations of MG1654 + 1346 (ref. 6) a radio quasar in near-perfect alignment with an elliptical galaxy, and show that radio emission is distorted into a narrow Einstein ring that lies within a diamond-shaped region bounded by caustic lines. From the details of the ring structure, a new model for the lensing geometry is deduced which leads to a more accurate estimate of the mass of the lensing galaxy. © 1990 Nature Publishing Group.