Very cold dust in galaxies

Abstract

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 +/- 2.8 K. The ratio of infrared luminosity to gas mass, L_IR/M_gas, where the latter quantity has been obtained from 1.3 mm observations within the central 11'' is about 90 L_sun/ M_sun. In contrast, the spectral energy distributions of inactive spirals require, apart from warm dust of 31.8 +/- 2.8 K, an additional very cold component of at most 12.9+/- 1.7 K. Determining the gas mass from 1.3 mm dust continuum maps that cover the optical extent of the inactive spirals we find L_IR/M_gas ~ 3 L_sun/ M_sun , a factor ~ 30 lower than for the active galaxies. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.