Subaru mid-infrared imaging of the quadruple lenses PG 1115+080 and B1422+231: Limits on substructure lensing

We present mid-infrared imaging at 11.7 mu m for the quadruple lens systems PG 1115+080 and B1422+231 using the cooled mid-infrared camera and spectrometer ( COMICS) attached on the Subaru Telescope. These lensed QSOs are characterized by their anomalous optical and radio flux ratios, as obtained for A1 and A2 images of PG 1115+080 and A, B, and C images of B1422+231, respectively, i.e., such flux ratios are hardly able to be reproduced by lens models with a smooth mass distribution. Our mid-infrared observations for these images have revealed that the mid-infrared flux ratio A2/A1 of PG 1115+080 is virtually consistent with smooth lens models (but inconsistent with the optical flux ratio), whereas for B1422+231, the mid- infrared flux ratios among the A, B, and C images are in good agreement with the radio flux ratios. We also identify a clear infrared bump in the spectral energy distributions of these QSOs, thereby indicating that the observed mid- infrared fluxes originate from a hot dust torus around a QSO nucleus. Based on the size estimate of the dust torus, we place limits on the mass of a substructure in these lens systems causing the anomalous optical or radio flux ratios. For PG 1115+ 080, the mass of a substructure inside an Einstein radius M-E is less than or similar to 16M(circle dot), corresponding to either a star or a low-mass CDM subhalo having a mass of M-100(SIS) less than or similar to 2.2 x 10(4) M-circle dot inside a radius of 100 pc if modeled as a singular isothermal sphere (SIS). For B1422+231, we obtain M-E greater than or similar to 209 M-circle dot, indicating that a CDM subhalo is more likely, having a mass of M-100(SIS) greater than or similar to 7.4 x 10(4) M-circle dot.