The origin of the far-infrared luminosity within the spiral galaxy M101

High resolution 60 and 100 μm images obtained with the Infrared Astronomical Satellite (IRAS) are compared with Ha images in order to investigate the origin of the far-infrared luminosity within the late-type spiral galaxy M101. There is a good correspondence between the far-infrared and Hα morphology. The far-infrared and Hα luminosities have been measured at 129 independent locations on the star forming disk of M101. After correcting the Ha luminosity for extinction and extrapolating the IRAS (40-120 μm) luminosity to 1000 μm we find that the far-infrared luminosity is commensurate with that expected from the O and B stars which are required to ionize the hydrogen gas, at all locations within M101. Additionally, the IRAS HiRes 60 and 100 μm images reveal that the dust temperature peaks coincide identically with the location of H II regions. The far-infrared luminosity of M101 is radiated primarily by dust with temperatures well in excess of that expected for cirrus, but similar to that observed for Galactic and extragalactic H II regions.