NEW EINSTEIN CROSS GRAVITATIONAL LENS CANDIDATES IN HUBBLE-SPACE-TELESCOPE WFPC2 SURVEY IMAGES

We report the serendipitous discovery of ''Einstein cross'' gravitatio nal lens candidates using the Hubble Space Telescope (HST). We have so far discovered two good examples of such lenses, each in the form of four faint blue images located in a symmetric configuration around a r ed elliptical galaxy. The high resolution of HST has facilitated the d iscovery of this optically selected sample of faint lenses with small (similar to 1 '') separations between the (I similar to 25-27) lensed components and the much brighter (I similar to 19-22) lensing galaxies . The sample has been discovered in the routine processing of HST fiel ds through the Medium Deep Survey pipeline, which fits simple galaxy m odels to broadband filter images of all objects detected in random sur vey fields using WFPC2. We show that the lens configuration can be mod eled using the gravitational field potential of a singular isothermal ellipsoidal mass distribution. With this model the lensing potential i s very similar, both in ellipticity and orientation, to the observed l ight distribution of the elliptical galaxy, as would occur when stars are a tracer population. The model parameters and associated errors ha ve been derived by two-dimensional analysis of the observed images. Th e maximum likelihood procedure iteratively converges simultaneously on the model for the lensing elliptical galaxy and the source of the len sed components. A systematic search is in progress for other gravitati onal lens candidates in the HST Medium Deep Survey. This should eventu ally lead to a good statistical estimate for lensing probabilities and enable us to probe the cosmological parameters.