Energy sources of the far-infrared emission of M33

We present observations of the spiral galaxy M33 with Spitzer at 24, 70, and 160 mum. The excellent resolution and mapping capabilities of Spitzer combined with the proximity of M33 result in observations that enable a detailed study of the distribution of star formation (SF) and dust in the galaxy. We compare the morphology of M33 at far-infrared wavelengths with other standard SF indicators such as Halpha and radio continuum using a Fourier filtering technique to separate the diffuse emission components from compact sources. We find that the infrared emission at 24 and 70 mum follows closely the structure of the ionized gas, indicating that it is heated largely by hot, ionizing stars. At 160 mum a diffuse cold dust component matches only approximately the structure of the old red stellar population or the distribution of blue light. It is, however, very similar to the structure of the diffuse nonthermal radio emission.