Properties of compact 250 μm emission and HII regions in M33 (HERM33ES)

Aims. Within the framework of the HERM33ES key program, using the high resolution and sensitivity of the Herschel photometric data, we study the compact emission in the Local Group spiral galaxy M33 to investigate the nature of the compact SPIRE emission sources. We extracted a catalogue of sources at 250 mu m in order to investigate the nature of this compact emission. Taking advantage of the unprecedented Herschel resolution at these wavelengths, we also focus on a more precise study of some striking H alpha shells in the northern part of the galaxy. Methods. We present a catalogue of 159 compact emission sources in M33 identified by SExtractor in the 250 mu m SPIRE band that is the one that provides the best spatial resolution. We also measured fluxes at 24 mu m and H alpha for those 159 extracted sources. The morphological study of the shells also benefits from a multiwavelength approach including H alpha, far-ultraviolet from GALEX, and infrared from both Spitzer IRAC 8 mu m and MIPS 24 mu m in order to make comparisons. Results. For the 159 compact sources selected at 250 mu m, we find a very strong Pearson correlation coefficient with the MIPS 24 mu m emission (r(24) = 0.94) and a rather strong correlation with the H alpha emission, although with more scatter (r(H alpha) = 0.83). The morphological study of the H alpha shells shows a displacement between far-ultraviolet, H alpha, and the SPIRE bands. The cool dust emission from SPIRE clearly delineates the H alpha shell structures. Conclusions. The very strong link between the 250 mu m compact emission and the 24 mu m and H alpha emissions, by recovering the star formation rate from standard recipes for HII regions, allows us to provide star formation rate calibrations based on the 250 mu m compact emission alone. The different locations of the H alpha and far-ultraviolet emissions with respect to the SPIRE cool dust emission leads to a dynamical age of a few Myr for the H alpha shells and the associated cool dust.