Gravitational lensing by dark galactic haloes

Abstract

Recent work on galaxy formation scenarios has raised the interesting possibility that there could be many galactic halo-like objects in which luminous galaxies have failed to form. Whether dark galactic halos which lack compact luminous components will reveal themselves by multiply imaging distant quasars is considered. A single such dark halo is unlikely to do so, but two dark halos coming along the line-of-sight can form multiple images of distance sources. If the density in such dark halos is a significant fraction of the closure density of the universe, such double halo lensing is found to be on the threshold of detectability. The double dark halo lensing scenario can naturally account for multiply-imaged systems with large image separations but no detectable lens. 2345 + 007, the lens system with the largest observed image separation of 7.3 arcsec, and 1635 + 267 are suggested to be good candidates for being quasars multiply imaged by two dark galactic haloes. It is also suggested that observation of the missing third image in these systems by, say, the Space Telescope could give credence to the operation of lensing by dark galactic haloes.