The population of young stellar clusters throughout the disk of M 33

Aims. The properties of young stellar clusters (YSCs) in M 33, identified from the center out to about twice the size of the bright star-forming disk, are investigated to determine possible spatial and time variations of the star formation process in this Local Group blue galaxy. Methods. 915 MIR sources have been extracted from the Spitzer 24 mu m image. Upon inspection of H alpha and GALEX images and exclusion of evolved AGB stars, a sample of 648 objects is selected as candidate YSCs and their luminosity function is examined. The spectral energy distribution of each object, based on aperture photometry, is compared with Starburst99 models to derive age, mass and AV of individual clusters. In the analysis we allow for different values of the upper mass cutoff of the stellar initial mass function (IMF), the porosity of the ISM, and the dustiness of HII regions. We also examine the influence of different dust models and include corrections for incompleteness of the IMF. Results. We find discrete MIR sources as far as the extent of the warped HI disk, i.e. 16 kpc from the galaxy center. Their surface density has a steep radial decline beyond 4.5 kpc, and flattens out beyond the optical radius at 8.5 kpc. We are able to identify YSCs out to 12 kpc. At large galactocentric radii, the paucity of luminous clusters and the relevance of hot dust emission become evident from the analysis of the bolometric and MIR luminosity functions. The YSC mass and size are correlated with a log-log slope of 2.09 +/- 0.01, similar to that measured for giant molecular clouds in M 33 and the Milky Way, which represent the protocluster environment. Most of the YSCs in our sample have A(V) similar to 0-1 mag and ages between 3 and 10 Myr. In the inner regions of M 33 the clusters span a wide range of mass (10(2) < M < 3 x 10(5) M-circle dot) and luminosity (10(38) < L-bol < 3 x 10(41) erg s(-1)), while at galactocentric radii larger than similar to 4 kpc we find a deficiency of massive clusters. Beyond 7 kpc, where the H alpha surface brightness drops significantly, the dominant YSC population has M < 10(3) M-circle dot and a slightly older age (10 Myr). This implies the occurrence of star formation events about 10 Myr ago as far as 10-12 kpc from the center of M 33. The cluster L-FUV vs. L-H alpha relation is non-linear for L-FUV < 10(39) erg s(-1), in agreement with randomly sampled models of the IMF which, furthermore, shows no appreciable variation throughout the M 33 disk.