The Lens Galaxy in Pg1115+080 Is an Ellipse

We use the structure of the Einstein ring image of the quasar host galaxy in the four-image quasar lens PG1115+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 non-ellipsoidal models are consistent with the ellipsoidal model. We find upper limits on the standard parameters for the deviation from an ellipse of |a3 /a0| < 0.035 and |a4 /a0| < 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 |∆x|< 0.005 on the offset between them. Neither the ellipsoidal nor the non-ellipsoidal 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 A1 and A2 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. Subject headings: cosmology: observations — dark matter — gravitational lensing — quasars: individual (PG 1115+080)