An infrared Einstein ring in the gravitational lens PG 1115+080

Hubble Space Telescope observations of the gravitational lens PG 1115+080 i n the infrared show the known z(l) = 0.310 lens galaxy and reveal the z(s) = 1.722 quasar host galaxy. The main lens galaxy G is a nearly circular (el lipticity epsilon < 0.07) elliptical galaxy with a de Vaucouleurs profile a nd an effective radius of R-e = 0." 59 + 0." 06 (1.7 +/- 0.2 h(-1) kpc for Omega(0) = 1 and h = H-0/100 km s(-1) Mpc(-1)). G is part of a group of gal axies that is a required component of all successful lens models. The new q uasar and lens positions (3 mas uncertainty) yield constraints for these mo dels that are statistically degenerate, but several conclusions are firmly established. (1) The principal lens galaxy is an elliptical galaxy with nor mal structural properties, lying close to the fundamental plane for its red shift. (2) The potential of the main lens galaxy is nearly round, even when not constrained by the small ellipticity of the light of this galaxy. (3) All models involving two mass distributions place the group component near the luminosity-weighted centroid of the brightest nearby group members. (4) All models predict a time delay ratio r(ABC) similar or equal to 1.3. (5) Our lens models predict H-0 = 44 +/- 4 km s(-1) Mpc(-1) if the lens galaxy contains dark matter and has a hat rotation curve and H-0 = 65 +/- 5 km s(- 1) Mpc(-1) if it has a constant mass-to-light ratio. (6) Any dark halo of t he main lens galaxy must be truncated near 1." 5 (4 h(-1) kpc) before the i nferred H-0 rises above similar to 60 km s(-1) Mpc(-1). (7) The quasar host galaxy is lensed into an Einstein ring connecting the four quasar images, whose shape is reproduced by the models. Improved near-infrared camera mult iobject spectrograph (NICMOS) imaging of the ring could be used to break th e degeneracy of the lens models.