2005ApJ...626...51Y

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^B₃/a₀|<0.035 and |a^B₄/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|Δx|<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 A₁and A₂quasar 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.