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Astronomy and Astrophysics


Context. Over the past few years several studies have provided estimates of the SFR (star–formation rate) or the total infrared luminosity from just one infrared band. However these relations are generally derived for entire galaxies, which are known to contain a large scale di use emission that is not necessarily related to the latest star–formation episode. Aims. We provide new relations to estimate the SFR from resolved star–forming regions at 100 m and 160 m. Methods. We select individual star–forming regions in the nearby (840 kpc) galaxy M33. We estimate the SFR combining the emission in H and at 24 m to calibrate the emission at 100 m and 160 m as SFR estimators, as mapped with PACS/Herschel. The data are obtained in the framework of the HERM33ES open time key project. Results. There is less emission in the HII regions at 160 m than at 100 m. Over a dynamic range of almost 2 dex in S FR we find that the 100 m emission is a nearly linear estimator of the SFR, whereas that at 160 m is slightly superlinear. Conclusions. The behaviour of individual star–forming regions is surprisingly similar to that of entire galaxies. At high S FR, star formation drives the dust temperature, whereas uncertainties and variations in radiation–transfer and dust–heated processes dominate at low S FR. Detailed modelling of both galaxies and individual star forming regions will be needed to interpret similarities and di erences between the two and assess the fraction of di use emission in galaxies.


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