THE DOUBLE QUASAR Q2138-431 - LENSING BY A DARK GALAXY

We report the discovery of a new gravitational lens candidate Q2138 - 431AB, comprising two quasar images at a redshift of 1.641, separated by 4.5 arcsec. The spectra of the two images are very similar, and the redshifts agree to better than 115 km s(-1). The two images have magn itudes B-J = 19.8 and 21.0, and, in spite of a deep search and image s ubtraction procedure, no lensing galaxy has been found with R < 23.8. Modelling of the system configuration implies that the mass-to-light r atio of any lensing galaxy is likely to be around 1000 M./L., with an absolute lower limit of 200 M./L. for an Einstein-de Sitter universe. We conclude that the most likely explanation of the observations is gr avitational lensing by a dark galaxy, although it is possible we are s eeing a binary quasar.