The Ha image is compared with a high resolution far infrared image obtained with the Infrared Astronomical Satellite (IRAS) in order to constrain the contribution of O and B stars to the far infrared luminosity of M81. The correspondence between the far infrared and Ha morphology is striking when both are convolved to a common resolution of 105 arc sec. The far infrared luminosity, L(FIR), and the Ha luminosity, L(Ha), has been measured at 85 independent locations within M81 and the histogram of L(FIR)/L(Ha) ratios is remarkably similar to that determined for Galactic HII regions. The results indicate that the majority, ~70%, of the far infrared and Ha luminosity of M81 is produced by high mass stars located in the star forming ring. The origin of the remaining 30% of the far infrared and Ha luminosity, that is co-extensive with the bulge, is ambiguous in that it could be powered equally well by young massive stars or old bulge stars.
More details appear in the Astronomical Journal, Devereux et al., 1995, Vol 110, 1115. See also the Astronomical Journal,Devereux et al., 1996, Vol 111, 2115 (Erratum).