DETECTION OF A COMPANION LENS GALAXY USING THE MID-INFRARED FLUX RATIOS OF THE GRAVITATIONALLY LENSED QUASAR H1413+117

We present the first resolved mid-infrared (IR) (11 mu m) observations of the four-image quasar lens H1413+117 using the Michelle camera on Gemini North. All previous observations (optical, near-IR, and radio) of this lens show a flux anomaly, where the image flux ratios cannot be explained by a simple, central lens galaxy. We attempt to reproduce the mid-IR flux ratios, which are insensitive to extinction and microlensing, by modeling the main lens as a singular isothermal ellipsoid. This model fails to reproduce the flux ratios. However, we can explain the flux ratios simply by adding to the model a nearby galaxy detected in the H band by the Hubble Space Telescope. This perturbing galaxy lies 4 ''.0 from the main lens and it has a critical radius of 0 ''.63 +/- 0 ''.02 which is similar to that of the main lens, as expected from their similar H-band fluxes. More remarkably, this galaxy is not required to obtain a good fit to the system astrometry, so this represents the first clear detection of an object through its effect on the image fluxes of a gravitational lens. This is a parallel to the detections of visible satellites from astrometric anomalies, and provides a proof of the concept of searching for substructure in galaxies using anomalous flux ratios.