On the metallicity dependence of the 24 mm luminosity as a star formation tracer

We investigate the use of the rest-frame 24 mu m luminosity as an indicator of the star formation rate (SFR) in galaxies with different metallicities by comparing it to the (extinction-corrected) H alpha luminosity. We carry out this analysis in two steps: First, we compare the emission from H (II) regions in different galaxies with metallicities between 12 + and 8.9. We find that the 24 mu m and the extinction-corrected Ha luminosities from individual H (II) log (O/H) = 8.1 regions follow the same correlation for all galaxies, independent of their metallicity. Second, the role of metallicity is explored further for the integrated luminosity in a sample of galaxies with metallicities in the range of 12 +. For this sample we compare the 24 mm and H alpha luminosities integrated over the entire galaxies log (O/ H) = 7.2-9.1 and find a lack of the 24 mu m emission for a given Ha luminosity for low-metallicity objects, likely reflecting a low dust content. These results suggest that the 24 mu m luminosity is a good metallicity-independent tracer for the SFR in individual H (II) regions. On the other hand, metallicity has to be taken into account when using the 24 mu m luminosity as a tracer for the SFR of entire galaxies.