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

Abstract

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 0A.644, di ering in brightness by only 0.4 mag. This system is almost certainly gravitationally lensed. 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 photometry for the two components: the spectrum shows no trace of any zero-redshift stellar absorption features belonging to a star with the temperature indicated by the broadband photometry. The optical spectrum shows strong absorption features of Mg II, Mg I, Fe II, Fe I, and Ca I, all at an identical intervening redshift of z\0.9313, probably due to the lensing object. The strength of Mg II and the presence of the other low-ionization absorption features is strong evidence for a damped Lya system, likely the disk of a spiral galaxy. Point-spread function Ðtting to remove the two quasar components 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 signiÐcantly redder than the fainter, due to either di erential extinction or microlensing.