Star formation in M 33: multiwavelength signatures across the disk

Aims. We use different tracers, such as Ha, ultraviolet (UV), and infrared (IR) emissions at various wavelengths, to study the dust and star formation (SF) conditions throughout the disk of M 33. Methods. We derive the radial distribution of dust, of the old and young stellar population using Spitzer and GALEX data, complemented by ground-based optical data and available surveys of atomic and molecular gas. We separate the contribution of discrete sources to the IR brightness from the diffuse emission. Results. At 8 and 24 mu m, discrete sources account for greater than or similar to 40% of the IR emission in the innermost 3 kpc, and for less than or similar to 20% further out. We find that stochastic emission from very small grains in the diffuse interstellar medium accounts for only similar to 10% of the diffuse 24 mu m emission, and that dusty circumstellar shells of unresolved, evolved AGB stars (carbon stars) are a viable alternative. The 8 mu m profile suggests that PAH emission declines faster with radius than the dust continuum. In annular regions 0.24 kpc wide, we find a mean extinction value for the stellar continuum A(V) similar to 0.25 mag with a weak dependence on radius, consistent with the shallow metallicity gradient observed. Dust opacity derived from the 160 mu m emission decreases instead by a factor of 10 from the center to edge of the star forming disk. Conclusions. Using extinction corrected UV and H alpha maps we find the global SF rate in M 33 over the last 100 Myr to be 0.45 +/- 0.10 M-circle dot yr(-1). Far-IR and total-IR luminosities can trace SF even though a high conversion factor is required to recover the effective rate. If carbon stars are powering the diffuse 24 mu m emission in M 33, this can trace star formation 1 Gyr ago and provide a more complete view of the SF history of the galaxy. Today the SF rate declines radially with a scale length of similar to 2 kpc, longer than for the old stellar population, suggesting an inside-out growth of the disk.