Polycyclic Aromatic Hydrocarbons as Star Formation Rate Indicators

As images and spectra from ISO and Spitzer have provided increasingly higher–fidelity representations of the mid–infrared (MIR) and Polycyclic Aromatic Hydrocarbon (PAH) emission from galaxies and galactic and extra–galactic regions, more systematic efforts have been devoted to establishing whether the emission in this wavelength region can be used as a reliable star formation rate indicator. This has also been in response to the extensive surveys of distant galaxies that have accumulated during the cold phase of the Spitzer Space Telescope. Results so far have been somewhat contradictory, reflecting the complex nature of the PAHs and of the mid–infrared–emitting dust in general. The two main problems faced when attempting to define a star formation rate indicator based on the mid–infrared emission from galaxies and star–forming regions are: (1) the strong dependence of the PAH emission on metallicity; (2) the heating of the PAH dust by evolved stellar populations unrelated to the current star formation. I review the status of the field, with a specific focus on these two problems, and will try to quantify the impact of each on calibrations of the mid–infrared emission as a star formation rate indicator.