2002ApJ...572..105S

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 µm galaxy sample–in total, 29 Seyfert 1 galaxies, 35 Seyfert 2 galaxies, and 12 starburst galaxies, or about half of the 12 µm active galaxy sample, plus 14 normal galaxies for comparison. ISO and Infrared Astronomical Satellite (IRAS) data are used to define 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 µm 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 µm, while, as already seen by IRAS, the former have flatter SEDs shortward of 60 µm. A mild anticorrelation is found between the [200-100] color and the ``60 µm excess.'' We infer that this is due to the fact that galaxies with a strong starburst component and thus a strong 60 µm 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 µm emission. This shifts the peak emission from around 150 µm in the most quiescent spirals to shorter than 60 µm 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 µm, the quiescent disk emission remains much cooler than the starburst component. Seyfert galaxies have 100-200 µm SEDs ranging from pure disks to pure starbursts, with no apparent contribution from their active nuclei at those wavelengths.