On the Origin of the 40-120 Micron Emission of Galaxy Disks: a Comparison with H-alpha Fluxes

A comparison of 40-120 micron IRAS fluxes with published Halpha and UBV photometry shows that the far infrared emission of galaxy disks consists of at least two components: a warm one associated with OB stars in HII-regions and young star-forming complexes, and a cooler one from dust in the diffuse, neutral interstellar medium, heated by the more general interstellar radiation field of the old disk population (a 'cirrus'-like component). Most spiral galaxies are dominated by emission from the cooler component in this model. A significant fraction of the power for the cool component must originate with non-ionizing stars. For a normal spiral disk there is a substantial uncertainty in a star formation rate derived using either the H-alpha or the far infrared luminosity.