The mid-infrared emission of local Luminous Infrared Galaxies

Abstract

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 - 10μm imaging observations of LIRGs have allowed us to spatially resolve the nuclear emission (star formation and/or AGN) and that of H II regions in the central 3 - 7kpc regions of LIRGs. From the comparison of the 8μm/Paα ratios of the integrated vs. resolved H II regions of LIRGs, we infer the existence of an 8μm diffuse component, not directly related to the ionizing stars, that can be as luminous as that from the resolved H II 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 H II regions alone. To this end we provide a calibration of the SFR in terms of the integrated 24μm luminosity that can be used for distant dusty galaxies.