1999A&A...351..495S

We present multi-filter far infrared photometry of active and inactive galaxies obtained with ISOPHOT. We find that the far infrared and submillimeter spectrum of the active galaxies can be described by a single modified black-body at a color temperature of 31.5±2.8K. The ratio of infrared luminosity to gas mass, L_IR/M_gas, where the latter quantity has been obtained from 1.3mm observations within the central 11'' is about 90L_☉/M_☉. In contrast, the spectral energy distributions of inactive spirals require, apart from warm dust of 31.8±2.8K, an additional very cold component of at most 12.9± 1.7K. Determining the gas mass from 1.3mm dust continuum maps that cover the optical extent of the inactive spirals we find L_IR/M_gas∼3L_☉/M_☉, a factor ∼30 lower than for the active galaxies.