The Lens Galaxy in PG 1115+080 is an Ellipse

We use the structure of the Einstein ring image of the quasar host galaxy in the four-image quasar lens PG 1115+080 to determine the angular structure of the gravitational potential of the lens galaxy. We find that it is well described as an ellipsoid and that the best-fit nonellipsoidal models are consistent with the ellipsoidal model. We find upper limits on the standard parameters for the deviation from an ellipse of | a / a 0 | < 0.035 and | a / a 0 | < 0.064. We also find that the position of the center of mass is consistent with the center of light, with an upper limit of |Δ | < 0 005 on the offset between them. Neither the ellipsoidal nor the nonellipsoidal models can reproduce the observed image flux ratios while simultaneously maintaining a reasonable fit to the Einstein ring, so the anomalous flux ratio of the A 1 and A 2 quasar images must be due to substructure in the gravitational potential such as compact satellite galaxies or stellar microlenses rather than odd angular structure in the lens galaxy.