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The dark halo of the main lens galaxy in QSO 0957 + 561

We present an analysis of infrared/optical/ultraviolet spectra of the two images of the first gravitationally lensed quasar Q0957 + 561A,B. The Hubble Space Telescope observations of Q0957 + 561A and Q0957 + 561B are separated in time by the known time delay in this system, so we can directly deduce the flux ratios. These flux ratios of images lead to important information on the dark halo of the main lens galaxy (a giant elliptical at redshift z = 0.36). Our measurements for the continuum are in good agreement with extinction in the elliptical galaxy and a small fraction of mass in collapsed objects (no need for gravitational microlensing). From the continuum and emission-line ratios, we also show evidence in favour of the existence of a network of compact dusty clouds. Ke yw ords: gravitational lensing ‐ dust, extinction ‐ galaxies: elliptical and lenticular, cD ‐ galaxies: haloes ‐ quasars: individual: QSO 0957 + 561 ‐ dark matter.