The mid-infrared emission of local Luminous Infrared Galaxies

In this paper we use the complementary imaging capabilities of Spitzer (sensitivity) and Gemini-South/T-ReCS (spatial resolution) to study the mid-IR properties of local (d < 75Mpc) LIRGs. The T-ReCS 8-10micron imaging observations of LIRGs have allowed us to spatially resolve the nuclear emission (star formation and/or AGN) and that of HII regions in the central 3-7kpc regions of LIRGs. From the comparison of the 8micron/Pa-alpha ratios of the integrated vs. resolved HII regions of LIRGs, we infer the existence of an 8micron diffuse component, not directly related to the ionizing stars, that can be as luminous as that from the resolved HII regions. We conclude that although the mid-IR integrated luminosity of galaxies undergoing dusty, intense star formation is a good indicator of the star formation rate (SFR), the empirical calibrations should be based on the integrated emission of nearby galaxies, not that of HII regions alone. To this end we provide a calibration of the SFR in terms of the integrated 24micron luminosity that can be used for distant dusty galaxies.