The host galaxy of the lensed quasar Q0957+561

Infrared images of the Q0957 + 561 gravitational lens obtained with the Hub ble Space Telescope show two large (similar to5 ") lensed images of the z(s ) = 1.41 quasar host galaxy. Parts of the host galaxy are doubly imaged, Li ke the quasar, while other parts are quadruply imaged. The distortions of t he host galaxy offer the best probe yet of the global structure of the lens ing potential, which is essential for determining the Hubble constant from the measured time delay. The distortions are inconsistent with the predicti ons of previously published lens models, which invalidates those models and their implications for H-0. New models show that the distortions finally b reak the long-standing degeneracy between the shape of the lens galaxy and the tidal shear contributed by the cluster containing the lens galaxy. The shape of the lens galaxy's mass distribution must be remarkably similar to the shape of its luminosity distribution, and most models that produce reas onable values for the Hubble constant roughly match the observed ellipticit y gradient and isophote twist of the lens galaxy. In addition, the cluster must be nonspherical and produce a relatively small tidal shear. Although t here are still degeneracies in the lens models that lead to a 25% uncertain ty in the derived value of the Hubble constant, there are also strong prosp ects for new observations to further improve the constraints and reduce the uncertainties.