UV spectra of the twin QSOs 0957+561 A, B

Abstract

Observations of the twin QSOs 0957+561 A and B by Walsh et al.¹ gave the same redshifts in both the emission line (z_e=1.41) and absorption line (z_a=1.39) systems and also showed similar spectral characteristics in the strengths of the emission lines, absorption lines and continua. This led to the proposal that they were one object, a double image of which was being formed by an intervening massive galaxy acting as a gravitational lens. More extended observations in the 6,000–6,900 Å range² and in the 4,000–7,000 Å region³ confirm the similarity of the two spectra and show very close agreement (14 kms⁻¹ r.m.s.) between the velocities of the absorption line systems, thus strengthening the lens hypothesis. Other data on the structure of the associated radio sources^4–6 are also consistent with the above proposition. An intervening galaxy has been looked for⁷ which may act as a gravitational lens: very deep red images were taken of the field using a CCD camera on the 200-inch Palomar telescope and supplemented by multichannel spectrophotometric scans of the double system. An elliptical galaxy of red magnitude 18.5 was detected which is nearly superimposed on the south (B) component of the twin QSO and which is the brightest member of a rich cluster of galaxies. The spectrophotometric data of the system show a contribution from this intervening galaxy and from the location of the characteristic Ca II 3,950 Å break, a redshift of 0.39 is estimated. Young et al.⁸ conclude that Q0957+561 A, B could be two images of the same object produced by this galaxy but that the lens properties are somewhat complex due to ‘aberrations’ induced by the rest of the cluster. We present here the first UV observations of Q0957+561 A, B and discuss them in terms of the lens hypothesis.