A galaxy which radiates strongly at 25 µm is likely to have an active nucleus. For Seyfert galaxies R=F (25 µm)/F(60 µm) is typically 0.2 to 0.5 (Miley & Neugebauer, 1985; De Grijp et al., 1985) and for quasars first detected by IRAS R ranges from 0.2 to 1.1 (Clowes, Leggett & Savage, 1991); whereas for IRAS galaxies for which star formation is the principal source of emission R is usually in the range 0.05 to 0.2. It appears that a ‘high’ value of R (» 0.2) strongly suggests the presence of an active nucleus while a ‘low’ value (R ≃ 0.2) does not exclude this. Hill, Becklin & Williams (1988) note that galaxies with high R tend to be compact at 10 µm which supports the idea that high R may be associated with nuclear activity.

The origin of the strong 25 µm emission from galaxies with active nuclei is unclear. Thermal emission from hot dust surrounding and heated by a power law source is an obvious possibility: however spherically symmetric models do not always provide good fits to the spectral energy distribution of Seyfert galaxies (Rowan-Robinson & Crawford 1989) and it appears that a disc geometry, perhaps combined with a high optical depth, may be needed in some cases. Other factors that may influence the models include the possible destruction of the very small grain component close to the AGN and the clumpiness of the dust distribution.