Hα, far-infrared and thermal radio continuum emission within the late-type spiral galaxy M33

New Hα imaging observations have revealed the morphology of the emission line gas and permitted the first measurement of the total Hα luminosity for the late-type spiral galaxy M33. The total Hα luminosity of M33 is (7.06±1.40) × 10⁶ L_⊙ and is dominated by emission from H II regions. The Hα image is compared with 6 and 20 cm thermal radio continuum images in order to quantify the extinction to H II regions in M33. The extinction is found to be high locally, but low globally. The extinction to the high surface brightness cores of H II regions corresponds to A_υ∼1 mag on average with no systematic dependence on radius. However, the extinction correction to the global Hα flux is much lower with A_υ∼0.0-0.4 mag. The difference suggests that the extinction is virtually negligible to the low surface brightness Hα emission outside the high surface brightness cores of H II regions. 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 M33. 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 840 independent locations within M33 and the histogram of L(FIR)/L(Hα) ratios is remarkably similar to that determined for Galactic H II regions. Approximately 70% of the far-infrared luminosity of M33 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 M33 is produced by massive stars.