Hα and far-infrared emission within the early type spiral galaxy M81

New Hα imaging observations have revealed the morphology of the emission-line gas and permitted the first measurement of the total Ha luminosity for the early type spiral galaxy M81. The total Hα luminosity of M81 is (1.7±0.5)X10⁷L_⊙. The Hα luminosity is dominated by emission from H II regions that are distributed approximately in an elliptical ring with a major axis diameter of 11 arcmin. A significant percentage, ∼17%, of the total Hα emission is associated with a curious feature in the central 3 arcmin of M81 that is best described as a nuclear spiral. A wide variety of sources that may be responsible for ionizing the nuclear spiral are evaluated including massive stars, old post-AGB stars, shocks, planetary nebulae, and x-rays. The Hα 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 Hα and far-infrared morphology is striking when both images are convolved to a common resolution of 105 arcsec. The far-infrared luminosity, L(FIR), and the Hα luminosity, L(Hα), have been measured at 85 independent locations within M81 and the histogram of L(FIR)/L(Hα) ratios is remarkably similar to that determined for Galactic H II regions. Approximately 75% of the far-infrared luminosity of M81 is radiated by dust with temperatures greater than expected for cirrus, but similar to the temperatures measured by IRAS for Galactic and extragalactic H II regions. The results indicate that the majority, ∼70%, of the far infrared and Hα 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 Hα 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.