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-IR (11 micron) 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 HST/NICMOS-NIC2. 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.
