High spatial resolution T-ReCS mid-infrared imaging of luminous infrared galaxies

We present diffraction-limited (FWHM similar to 0."3) Gemini/T-ReCS mid-infrared (MIR; N-band or narrowband at 8.7 mu m) imaging of four luminous infrared galaxies (LIRGs) drawn from a representative local sample. The MIR emission in the central few kiloparsecs is strikingly similar to that traced by Pa alpha and generally consists of bright nuclear emission and several compact circumnuclear and/ or extranuclear H II regions. The central MIR emission is dominated by these powerful H II regions, consistent with the majority of active galactic nuclei in this local sample of LIRGs contributing a minor part of the MIR emission. The luminous circumnuclear H II regions detected in LIRGs follow the extrapolation of the 8 mu m versus Pa alpha relation found for M51 H II knots. The integrated central 3-7 kpc of galaxies, however, present elevated 8 mu m/Pa alpha ratios with respect to individual H II regions, similar to the integrated values for star-forming galaxies. Our results show that the diffuse 8 mu m emission, not directly related to the ionizing stellar population, can be as luminous as that from the resolved H II regions. Therefore, calibrations of the star formation rate for distant galaxies should be based on the integrated 8 mu m emission of nearby galaxies, not that of the H II regions alone.