A Gravitational Lens Candidate Behind The Fornax Dwarf Spheroidal Galaxy

Extract

CCD astrometry makes possible milli-arcsecond precision relative astrometry at faint magnitudes (Tinney 1994a). The measurement of proper motions for the nearest galaxies is therefore a project requiring only years – not decades. A prerequisite for such measurements, however, is a set of unresolved, extragalactic reference objects – ie. QSOs. While searching for such objects behind the Fornax dSph galaxy, Tinney (1995b) discovered the QJ0240-343AB system as a pair of bright, UV-excess objects (U-B = −1.0, B = 19.0 & 19.8) with a separation of 6.1″. Spectra obtained with the 3.9 m Anglo-Australian telescope and the RGO+FORS spectrographs (Fig. 1) have shown both components of the pair to be at a redshift of z = 1.4, with a rest velocity difference consistent with zero at a 1-σ limit of 180 km/s. The spectra also show a definite metal-line absorption system at z = 0.543 and a possible system at z = 0.337. No bright lens is seen, but the strong similarities in the spectra suggest a gravitational lens nature.