2007A&A...476.1161V

Combining the relative vicinity of the Local Group spiral galaxy M33 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. The mid- and far-IR emission of M33 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 24µm) of each type of object. Moreover, we generated a catalogue of 24µm sources and inferred their nature from the observed and theoretical colours of the known type sources. We estimated the star formation rate in M33 both globally and locally, from the IR emission and from the Hα emission line. 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 ∼600K, and moderate extinction A_V≲10. The 24µm 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 5x10³⁷erg/s, 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 24µm luminosities within the central 5kpc of M33 are comparable and of order 4x10²⁸erg/s/Hz (νL_ν(8)=1.5x10⁴² and νL_ν(24)=4.4x10⁴¹erg/s). We estimate the total IR emission in the same region of M33 to be 10⁹L_☉. The discrete sources account for about one third of the 24 µm emission while the rest is diffuse. From the IR emission, we derive a star formation rate for the inner disk equal to 0.2M_☉/yr, consistent with the star formation rate obtained from the Hα emission.