Hubble Space Telescope Wide Field Camera Imaging of the Gravitational Lens 2237+0305

Abstract

Hubble Space Telescope Wide Field Camera images of the gravitational lens 2237+0305 in the F336W ("U") and F702W ("A") filters are used to construct models of the system. Positions for the four quasar images, accurate to +/-0.015", and relative magnitudes in U and R, accurate to +/-0.06 and 0.04 mag respectively, are determined. The upper limits on the observed brightness of the fifth image are found to be <~7% of the brightest quasar image. The long "color baseline," from R to U, permits good estimates of the differential image reddening. With M/L as the only free parameter describing the lens and two further parameters describing the position of the quasar in the source plane, the eight quasar image positions can be reproduced to within ~0.03" (or 2.5% of their separations); at the same time the three relative brightnesses can be reproduced to within factors of 1.5. The total image amplification is well constrained by these models: a factor of 18.5+/-2. This modeling assumes only that the surface mass density of the lens galaxy is proportional to the observed surface brightness. The mass of the lens inside 0.9" is found to be 1.08+/-0.02X10^10^M_sun_h^-1^_100_ corresponding to a mass-to-light ratio in B of 12.3h_100_. This M/L estimate agrees with values obtained from stellar dynamics for other elliptical galaxies. The mass in the inner 500h^-1^_100_ pc is constrained by these phenomenological models to within +/-2%, a more precise mass determination than for any other known bulge or elliptical. A comparison of predictions from this mass model with the measured central velocity dispersion yields a distance independent agreement to within 10%, assuming isotropic velocity dispersions. The influence of the mass associated with the Mg II absorber at z = 0.9 on the lensing appears to be unimportant. The detailed agreement of the lens model with the observations of 2237+0305, achieved by using the lens equations of general relativity and the cosmological interpretation of quasar redshifts, provides confirmation of these standard theories.