  With appropriate spatial resolution, images of spiral galaxies in thermal
infrared (~10 micron and beyond) often reveal a bright central component,
distinct from the stellar bulge, superimposed on a disk with prominent spiral
arms. ISO and Spitzer studies have shown that much of the scatter in the
mid-infrared colors of spiral galaxies is related to changes in the relative
importance of these two components, rather than to other modifications, such as
the morphological type or star formation rate, that affect the properties of
the galaxy as a whole. With the Herschel imaging capability from 70 to 500
micron, we revisit this two-component approach at longer wavelengths, to see if
it still provides a working description of the brightness distribution of
galaxies, and to determine its implications on the interpretation of global
far-infrared properties of galaxies.
