Star formation in M 33:: Spitzer photometry of discrete sources

Aims. Combining the relative vicinity of the Local Group spiral galaxy M 33 with the Spitzer images, we investigate the properties of infrared (IR) emission sites and assess the reliability of the IR emission as a star formation tracer. Methods. The mid-and far-IR emission of M 33 was obtained from IRAC and MIPS images from the Spitzer archive. We compared the photometric results for several samples of three known types of discrete sources (HII regions, supernovae remnants and planetary nebulae) with theoretical diagnostic diagrams, and derived the spectral energy distribution (from 3.6 to 24 mu m) of each type of object. Moreover, we generated a catalogue of 24 mu m sources and inferred their nature from the observed and theoretical colours of the known type sources. We estimated the star formation rate in M 33 both globally and locally, from the IR emission and from the Ha emission line. Results. The colours of the typical IR emissions of HII regions, supernovae remnants and planetary nebulae are continuous among the different samples, with overlapping regions in the diagnostic diagrams. The comparison between the model results and the colours of HII regions indicates a dusty envelope at relatively high temperatures similar to 600 K, and moderate extinction A(V) less than or similar to 10. The 24 mu m sources IR colours follow the regions observationally defined by the three classes of known objects but the majority of them represent HII regions. The derived total IR luminosity function is in fact very similar to the HII luminosity function observed in the Milky Way and in other late type spirals. Even though our completeness limit is 5 x 10(37) erg s(-1), in low density regions we are able to detect sources five times fainter than this, corresponding to the faintest possible HII region. The 8 and 24 mu m luminosities within the central 5 kpc of M 33 are comparable and of order 4 x 10(28) erg s(-1) Hz(-1) (vL(v)(8) = 1.5 x 10(42) and vL(v)(24) = 4.4 x 10(41) erg s(-1)). We estimate the total IR emission in the same region of M 33 to be 10(9) L-circle dot. The discrete sources account for about one third of the 24 mu m emission while the rest is diffuse. From the IR emission, we derive a star formation rate for the inner disk equal to 0.2 M-circle dot yr(-1), consistent with the star formation rate obtained from the Ha emission.