Star formation in M 33 : multiwavelength signatures across the disk

Aims. We use different tracers, such as Hα, 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 μm, discrete sources account for & 40% of the IR emission in the innermost 3 kpc, and for . 20% further out. We find that stochastic emission from very small grains in the diffuse interstellar medium accounts for only ∼10% of the diffuse 24 μm emission, and that dusty circumstellar shells of unresolved, evolved AGB stars (carbon stars) are a viable alternative. The 8 μm 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 stellar continuum AV ∼ 0.25 mag with a weak dependence on radius, consistent with the shallow metallicity gradient observed. Dust opacity derived from the 160 μm emission decreases instead by a factor 10 from the center to edge of the SF disk. Conclusions. Using extinction corrected UV and Hα maps we find the global SF rate in M 33, over the last 100 Myr, to be 0.45±0.10 M⊙ yr−1. FIR (far-IR) and TIR (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 μm 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 ∼ 2 kpc, longer than for the old stellar population, suggesting an inside-out growth of the disk.