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

Abstract

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 HII regions. The Hα image is compared with 6 and 20 cm thermal radio continuum images in order to quantify the extinction to HII regions in M33. The extinction is found to be high locally, but low globally. The extinction to the high surface brightness cores of HII regions corresponds to Av∼1 magnitude on average with no systematic dependence on radius. However, the extinction correction to the global Hα flux is much lower with Av∼0.00 to 0.4 magnitudes. The difference suggests that the extinction is virtually negligible to the low surface brightness Hα emission outside the high surface brightness cores of HII 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 arc seconds. 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 remarkably similar to that determined for Galactic HII 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 HII regions. The results indicate that the majority (⩾70%) of the far infrared and Hα luminosity of M33 is produced by massive stars.