New Mid-Infrared Diagnostic of the Dusty Torus Model for Seyfert Nuclei

We propose a new diagnostic of the "dusty torus" model for Seyfert nuclei. Dust grains in the torus are heated by the nuclear continuum and reradiate mostly in the mid-infrared wavelengths. From the geometry of the torus, it is predicted that the emission at λ ≤ 10 μm has strong dependence on the viewing angle. Since the dependence is predicted to be insignificant at λ ≥ 10 μm, we study the flux ratio between 3.5 μm (L band) and 25 μm; R(L, 25) = log[(ν3.5 μmS)/(ν25 μmS)]. In three different samples (optically selected, X-ray-selected, and infrared-selected samples) of Seyfert galaxies, the observed values of R(L, 25) between type 1 Seyfert galaxies (S1's) and type 2 Seyfert galaxies (S2's) are found to be clearly separated; R(L, 25) > -0.6 for S1's while R(L, 25) < -0.6 for S2's. This implies universality of their torus properties. With this result and the other observational characteristics, we investigate the most plausible torus model among those presented in Pier & Krolik.