The Double Quasar Q2138-431: Lensing by a Dark Galaxy?
Abstract
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 arcsecs. The spectra of the two images are very similar, and the redshifts agree to better than 115 km.sec-1. The two images have magnitudes BJ = 19.8 and BJ = 21.0, and in spite of a deep search and image subtraction procedure, no lensing galaxy has been found with R < 23.8. Modelling of the system configuration implies that the mass-to-light ratio 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 gravitational lensing by a dark galaxy, although it is possible we are seeing a binary quasar.
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