Very cold dust in galaxies

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, LIR/Mgas, where the latter quantity has been obtained from 1.3 mm 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 1.3 mm dust continuum maps that cover the optical extent of the inactive spirals we find LIR/Mgas ∼ 3L / M , a factor ∼ 30 lower than for the active galaxies.