Program: GS-2008B-C-2

We propose high-spatial-resolution mid-infrared 20 micron imaging observations of nearby ultraluminous infrared galaxies (ULIRGs) at z ~ 0.1, for which our previous infrared spectroscopy revealed that their energy sources are more centrally concentrated than the surrounding dust. Although these ULIRGs are naturally explained by buried AGNs (powered by compact mass-accreting supermassive blackholes), rather than normal starbursts (stellar energy sources and dust are spatially mixed), exceptionally centrally-concentrated starbursts remain a possibility. Through the proposed 20 micron imaging observations, we estimate the spatial extent of the 20 micron dust emission from these ULIRG's cores, using the diffraction-limited imaging capability of Gemini-S telescope at 20 micron. Our scientific goal is to quantitatively confirm that the emission surface brightnesses of the energy sources substantially exceed the maximum value achieved by starburst phenomena (energy generation efficiency ~0.5% * mc^2), and thereby to obtain evidence that the energy sources must be buried AGNs (energy generation efficiency ~6-40% * mc^2). The high sensitivity and high spatial resolution (at ~ 20 micron) of a ground-based 8 m telescope at a dry site are crucial for the success of this experiment.