THE ORIGIN OF THE FAR-INFRARED LUMINOSITY WITHIN THE SPIRAL GALAXY M101

High resolution 60 and 100 mu m 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 Hac morphology. The far-infrared and Her luminosities have been measured at 129 independent locations on the star forming disk of M101. After correcting the Her luminosity for extinction and extrapolating the IRAS (40-120 mu m) luminosity to 1000 mu m 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 mu m 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.