ISO long wavelength spectrograph observations of cold dust in galaxies

We describe observations of five nearby galaxies obtained using the Long Wa velength Spectrograph (LWS) on the Infrared Space Observatory (ISO). We obs erved five galaxies, using spectrograph apertures positioned at the galacti c center and in the outskirts of the disk, to compare the spectral energy d istribution of the emitting dust at different positions. The central spectr a are typical of those inferred previously from IRAS data; peaking at about 100 mum with estimated dust temperatures of T-d = 30-35 K. However, there is a rapid change in the spectral energy distribution with increasing galac tocentric distance. In the outer regions the spectra are flat, or still ris ing, out to 197 mum, indicating a predominantly cold dust component with T- d < 20 K. In the central regions, the inferred cold dust component is 2-10 times more massive than the warm dust, and it increasingly dominates the ma ss and spectral energy distribution in the outer regions. We discuss reason s for believing that emission beyond <approximate to>150 mum in disk galaxi es is associated with a separate component of dust with an extended distrib ution that may be associated with a possible molecular halo.