The genesis of starbursts and infrared emission in the centers of galaxies

We present 10.8-micron maps of the central regions of 21 IR-luminous galaxies. Each map, obtained using the NASA MSFC bolometer array at the IRTF, has a resolution of about 4 arcsec and spans typically 25 x 20 arcsec. Some of the galaxies have also been mapped at 12.5, 19.2, and 30/microns. The sample consists primarily of starburst galaxies but includes several Seyferts and LINERs. On average, the flux detected in each map is about 58 percent of that detected at 12 microns by IRAS. We see a variety of morphologies, including compact and elongated sources and more complex distributions exhibiting such features as kiloparsec-size rings and double lobes. The mid-IR radiation is emitted by dust particles, and for two of the galaxies we see spatial color variations attributable to the presence of very small grains and consistent with their depletion in the starbursts. The mid-IR emission traces the dust heated by the youngest stars. Comparison of the distributions of intense star formation and the dense neutral interstellar medium for eight of these galaxies demonstrates that the two are intimately linked.