ISOLong Wavelength Spectrograph Observations of Cold Dust in Galaxies

We describe observations of five nearby galaxies obtained using the Long Wavelength Spectrograph (LWS) on the Infrared Space Observatory (ISO). We observed five galaxies, using spectrograph apertures positioned at the galactic center and in the outskirts of the disk, to compare the spectral energy distribution of the emitting dust at different positions. The central spectra are typical of those inferred previously from IRAS data; peaking at about 100 μm with estimated dust temperatures of T(d) = 30-35 K. However, there is a rapid change in the spectral energy distribution with increasing galactocentric distance. In the outer regions the spectra are flat, or still rising, out to 197μm, 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 mass and spectral energy distribution in the outer regions. We discuss reasons for believing that emission beyond ≈150 μm in disk galaxies is associated with a separate component of dust with an extended distribution that may be associated with a possible molecular halo.