8-13 m spectrophotometry of galaxies - IV. Six more Seyferts and 3C 345

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Abstract
8–13 µm spectra of a further six Seyfert nuclei and the quasar 3C 345 are presented, together with those of 4 more starburst nuclei in a companion paper. With other published work such spectra now exist for 21 active and 13 starburst nuclei, and this division is clearly distinguished by the form of their 10-µm spectra. The active galaxies are typified by power-law spectra with a tendency for the spectral index to steepen with the progression quasar–Seyfert 1–Seyfert 2 and with silicate absorption sometimes occurring in Seyfert 2's. In contrast the starburst galaxy spectra are dominated by emission from the unidentified dust-associated features and frequently show the [Ne II] fine structure line. It is argued that the tiny grains thought to be responsible for the unidentified features (Sellgren) cannot survive the hard radiation field of the nuclear power-law source and that there are two components of the infrared emission: a non-thermal component associated directly with the nuclear power-law source and a thermal component due to dust grains of size >0.01 µm distributed in the narrow line regions and heated directly by the power-law source. In quasars the non-thermal component dominates while dust emission plays a progressively larger role with diminishing nuclear activity. Silicate emission, typical of luminous Galactic sources, is conspicuously absent from active galaxy nuclei; either these grains are a much less important fraction of the grain population or they are much less efficiently heated.

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