The far-infrared energy distributions of Seyfert and starburst galaxies in the local universe:: Infrared Space Observatory photometry of the 12 micron active galaxy sample

New far-infrared photometry with ISOPHOT aboard the Infrared Space Observatory (ISO) is presented for 58 galaxies with homogeneous published data for another 32 galaxies, all belonging to the 12 mum galaxy sample in total, 29 Seyfert 1 galaxies, 35 Seyfert 2 galaxies, and 12 starburst galaxies, or about half of the 12 mum active galaxy sample, plus 14 normal galaxies for comparison. ISO and Infrared Astronomical Satellite ( IRAS) data are used to de ne color-color diagrams and spectral energy distributions (SEDs). Thermal dust emission at two temperatures (one cold at 15-30 K and one warm at 50-70 K) can fit the 60-200 mum SED, with a dust emissivity law proportional to the inverse square of the wavelength. Seyfert 1 galaxies and Seyfert 2 galaxies are indistinguishable longward of 100 mum, while, as already seen by IRAS, the former have flatter SEDs shortward of 60 mum. A mild anticorrelation is found between the [200-100] color and the "60 mum excess. We infer that this is due to the fact that galaxies with a strong starburst component and thus a strong 60 mum flux have a steeper far-infrared turnover. In non-Seyfert galaxies, increasing the luminosity corresponds to increasing the star formation rate, which enhances the 25 and 60 mum emission. This shifts the peak emission from around 150 mum in the most quiescent spirals to shorter than 60 mum in the strongest starburst galaxies. To quantify these trends further, we identified with the IRAS colors three idealized infrared SEDs: pure quiescent disk emission, pure starburst emission, and pure Seyfert nucleus emission. Even between 100 and 200 mum, the quiescent disk emission remains much cooler than the starburst component. Seyfert galaxies have 100-200 mum SEDs ranging from pure disks to pure starbursts, with no apparent contribution from their active nuclei at those wavelengths.