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Subaru Mid-infrared Imaging of the Quadruple Lenses. II. Unveiling Lens Structure of MG0414+0534 and Q2237+030

We present mid-infrared imaging at 11.7 $μ$m for the quadruple lens systems, MG0414+0534 and Q2237+030, using the cooled mid-infrared camera and spectrometer (COMICS) attached on the Subaru telescope. MG0414+0534 is characterized by a bright pair of lensed images (A1, A2) and their optical flux ratio A2/A1 deviates significantly from the prediction of a smooth lens model. Q2237+030 is comprised of four lensed images, which are significantly affected by microlensing in a foreground lensing galaxy. Our mid-infrared observations of these lensed images have revealed that the mid-infrared flux ratio for A2/A1 of MG0414+0534 is nearly unity (0.90 +- 0.04). We find that this flux ratio is systematically small, at 4 to 5 sigma level, compared with the prediction of a best smooth lens model (1.09) represented by a singular isothermal ellipsoid and external shear, thereby suggesting the presence of substructures to explain our observational result. In contrast, for Q2237+030, our high signal-to-noise observation indicates that the mid-infrared flux ratios between all the four images of Q2237+030 are virtually consistent with the prediction of a smooth lens model. Based on the size estimate of the dust torus surrounding the nuclei of these QSOs, we set limits on the mass of a substructure in these lens systems, which can cause anomalies in the flux ratios. For MG0414+0534, since the required mass of a substructure inside its Einstein radius is > 360 M_sun, millilensing by a CDM substructure is most likely. If it is modeled as a singular isothermal sphere, the mass inside radius of 100 pc is given as > 1.0x10^5 M_sun. For Q2237+030, there is no significant evidence of millilensing, so the reported anomalous flux ratios in shorter wavelengths are entirely caused by microlensing by stars (abridged).