H-ALPHA, FAR-INFRARED AND THERMAL RADIO-CONTINUUM EMISSION WITHIN THELATE-TYPE SPIRAL GALAXY M33

New H alpha imaging observations have revealed the morphology of the e mission line gas and permitted the first measurement of the total H al pha luminosity for the late-type spiral galaxy M33. The total H alpha luminosity of M33 is (7.06+/-1.40) x 10(6) L. and is dominated by emis sion from H II regions. The H alpha 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 upsilon similar to 1 mag on average with no systematic dependence on radius. However, the extinction correction to the global H alpha flux is much lower with A(upsilon) similar to 0 .0-0.4 mag. The difference suggests that the extinction is virtually n egligible to the low surface brightness Her emission outside the high surface brightness cores of H II regions. The H alpha image is compare d with a high-resolution far-infrared image, obtained with the Infrare d Astronomical Satellite (IRAS), in order to constrain the contributio n of O and B stars to the far-infrared luminosity of M33. The correspo ndence between the H alpha and far-infrared morphology is striking whe n both images are convolved to a common resolution of 105 arcsec. The far-infrared luminosity, L(FIR), and the H alpha luminosity, L(H alpha ), have been measured at 840 independent locations within M33 and the histogram of L(FIR)IL(H alpha) ratios is remarkably similar to that de termined for Galactic H II regions. Approximately 70% of the far-infra red luminosity of M33 is radiated by dust with temperatures greater th an expected for cirrus, but similar to the temperatures measured by IR AS for Galactic and extragalactic H II regions. The results indicate t hat the majority (greater than or equal to 70%) of the far infrared an d H alpha luminosity of M33 is produced by massive stars. (C) 1997 Ame rican Astronomical Society.