A close-separation double quasar lensed by a gas-rich galaxy

In the course of a Cycle 8 snapshot imaging survey with the Space Telescope Imaging Spectrograph (STIS), we have discovered that the z = 1.565 quasar HE 0512-3329 is a double with image separation 0 ".644, differing in bright ness by only 0.4 mag. This system is almost certainly gravitationally lense d. Although separate spectra for the two images have not yet been obtained, the possibility that either component is a Galactic star is ruled out by a high signal-to-noise composite ground-based spectrum and separate photomet ry for the two components: the spectrum shows no trace of any zero-redshift stellar absorption features belonging to a star with the temperature indic ated by the broadband photometry. The optical spectrum shows strong absorpt ion features of Mg II, Mg I, Fe II, Fe I, and Ca I, all at an identical int ervening redshift of z = 0.9313, probably due to the lensing object. The st rength of Mg II and the presence of the other low-ionization absorption fea tures is strong evidence for a damped Ly alpha system, likely the disk of a spiral galaxy. Point-spread function fitting to remove the two quasar comp onents from the STIS image leads to a tentative detection of a third object , which may be the nucleus of the lensing galaxy. The brighter component is significantly redder than the fainter, due to either differential, extinct ion or microlensing.