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 lambda less than or equal to 10 mu m has strong dependence on the viewing angle. Since the dependence is predicted to be insignificant at lambda greater than or equal to 10 mu m, we study the flux ratio between 3.5 mu m (L band) and 25 mu m; R(L, 25) = log [(v(3.4 mu m) S-v3.5 mu m)/(v(25 mu m) Sv(25 mu m))]. 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.