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Published online by Cambridge University Press: 30 March 2011
As images and spectra from ISO and Spitzer have provided increasingly higher–fidelityrepresentations of the mid–infrared (MIR) and Polycyclic Aromatic Hydrocarbon (PAH)emission from galaxies and galactic and extra–galactic regions, more systematic effortshave been devoted to establishing whether the emission in this wavelength region can beused as a reliable star formation rate indicator. This has also been in response to theextensive surveys of distant galaxies that have accumulated during the cold phase of theSpitzer Space Telescope. Results so far have been somewhat contradictory, reflecting thecomplex nature of the PAHs and of the mid–infrared–emitting dust in general. The two mainproblems faced when attempting to define a star formation rate indicator based on themid–infrared emission from galaxies and star–forming regions are: (1) the strongdependence of the PAH emission on metallicity; (2) the heating of the PAH dust by evolvedstellar populations unrelated to the current star formation. I review the status of thefield, with a specific focus on these two problems, and will try to quantify the impact ofeach on calibrations of the mid–infrared emission as a star formation rate indicator.