  The Sc galaxy M33 has been mapped with ISOPHOT in the far-infrared, at 60,
100, and 170mue. 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 ~45K, and
a cold one, at ~16K. 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 H-alpha/FIR flux ratio increases significantly with the
distance from the galaxy center, probably due to decreasing extinction. An
anti-correlation of F_Ha/F_60 with F_170 suggests the intrinsic extinction to
be related to the cold dust surface brightness according to
A_V/S_170~0.03mag/MJy*sr. For the total galaxy the star formation rate (SFR)
derived from the FIR is in agreement with that derived from the de-extincted
H-alpha 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.
