New constraints on the unified model of Seyfert galaxies

We present new 10 μm (N-band) photometry for 70 Seyfert galaxies, 43 of them previously unobserved. These observations, together with those collected from the literature, complete the 10 μm photometry for the CfA Sy galaxies and cover 80% of the Sy found in the RSA and 70% of the Sy in the IRAS 12 μm sample. From this data set, we find that Sy not showing any evidence for broad lines are systematically weaker in 10 μm nuclear emission than Sy nuclei having broad lines. This result may indicate the existence of a group of very low-luminosity Sy2 galaxies that do not have Sy1 counterparts in equal numbers, contrary to the strict unified theory. Alternately, the result can be reconciled with unified theories if a specific type of geometry is assumed for the circumnuclear obscuring material. By comparing the 10 μm ground-based observations with the IRAS 12 μm fluxes, we also study the properties of the extended mid-IR emission, i.e., the star forming activity of the host galaxy of the Sy nucleus. We find Sy2 to lie preferentially in galaxies experiencing enhanced star-forming activity, while Sy1 lie in normal or quiescent galaxies. This result appears to be inconsistent with the strict unified model, since the host galaxy properties should be independent of the orientation of a circumnuclear torus and therefore should be independent of nuclear type. Our finding could be explained by adding to the unified model a link between starforming activity and the amount of obscuring material collected in the circumnuclear region.