High-resolution mid-infrared imaging of infrared-luminous starburst galaxies

Observations for seven infrared-luminous starburst galaxies are reported in the mid-infrared from 8 to 18 mum using the Keck telescopes with spatial resolution approaching the diffraction limit. All of the galaxies observed show evidence of strong interactions based on optical morphologies. For these galaxies, a substantial fraction, usually more than 50%, of the infrared luminosity is generated in regions ranging in sizes from 100 pc to 1 kpc. Nuclear starbursts often dominate the infrared luminosity, but this is not always true. In some galaxies, most notably NGC 6090, substantial infrared luminosity greatly in excess of the nuclear luminosity is generated in regions associated with the physical interaction between two galaxies. The radio emission is a good tracer of the location of high-luminosity young stars. The visual/ultraviolet radiation output of the nearby star-forming galaxies is dominated by emission from regions that are generally not producing the copious infrared luminosity of the systems. As seen in comparing the mid-infrared and near-infrared images of the galaxies observed here, the regions of high-infrared luminosity in local galaxies are significantly smaller than the galaxies as a whole. The integrated spectral energy distributions (SEDs) of these galaxies are very different from the SEDs of the regions of star formation. If the SEDs of star-forming regions in these galaxies reflect the SEDs that would be found in forming galaxies at high redshift, we would expect the distant galaxies to be dominated by the mid- and far-infrared luminosity output far more than the integrated luminous output of nearby starburst galaxies would suggest.