The spiral galaxy M33 mapped in the FIR by ISOPHOT -: A spatially resolved study of the warm and cold dust

The Sc galaxy M 33 has been mapped with ISOPHOT in the far-infrared, at 60, 100, and 170 mum. The spatial resolution of these FIR maps allows the separation of spiral arms and interarm regions and the isolation of a large number of star-forming regions. The spectral energy distribution in the FIR indicates a superposition of two components, a warm one originating from dust at similar to45 K, and a cold one, at similar to16 K. The warm component is concentrated towards the spiral arms and the star-forming regions, and is likely heated by the UV radiation from OB stars. The cold component is more smoothly distributed over the disk, and heated by the diffuse interstellar radiation. For the about 60 star-forming regions detected the Halpha/FIR flux ratio increases significantly with the distance from the galaxy center, probably due to decreasing extinction. An anti-correlation of F-Halpha/F-60 with F-170 suggests the intrinsic extinction to be related to the cold dust surface brightness according to A(V)/S-170 similar to 0.03 mag MJy(-1) sr. For the total galaxy the star formation rate (SFR) derived from the FIR is in agreement with that derived from the de-extincted Halpha emission. For individual star-forming regions, a consistency between SFRs derived from the optical and from the FIR requires only a fraction of the UV radiation to be absorbed locally. The individual star-forming regions also show a local radio-FIR correlation. This local correlation is, however, due to quite different components than to those that lead to the well-known global radio-FIR correlation for entire galaxies.