Optical and far-infrared emission of IRAS Seyfert galaxies

Abstract

This paper presents an analysis of moderately large samples of type 1 and 2 Seyfert galaxies through optical observations and far-infrared IRAS data, also taking into account theoretical color indices derived from dust emission models. The galaxies in the samples cover a rather large interval in far infrared luminosity, i.e., 7.6 ≤log (LIR/Lʘ)≤12.6. We show that both types of Seyferts have approx- imately the same distribution of number of objects with a given LIR. Galaxies with similar far-infrared color indices α(100, 60) are grouped together, and the corresponding average color indices are interpreted in terms of a simple model in which the observed colors result from the combination of dust directly heated by the active galactic nucleus with a component from the host galaxy represented by the emission of cool dust. On the basis of the average IRAS colors of the derived groups, we show that type 1 and 2 /L Seyfert galaxies are undistinguishable from each other. From the luminosity ratios LIR/Ha and LIR/L[O III], we show that basically the same model can be applied to both types of Seyfert, only allowing for the variation of model conditions : type 2 Seyferts would be like type 1 Seyferts but with the Seyfert nucleus and broad line region more efectively "hidden" by dust.