Toward an understanding of the mid-infrared surface brightness of normal galaxies

We report a mid-infrared color and surface brightness analysis of IC 10, NG C 1313, and NGC 6946, three of the nearby galaxies studied under the Infrar ed Space Observatory Key Project on Normal Galaxies. Images with less than 9 " (170 pc) resolution of these nearly face-on, late-type galaxies were ob tained using the LW2 (6.75 mu m) and LW3 (15 mu m) ISOCAM filters. Although their global flux ratios are similar and typical of normal galaxies, they show distinct trends of this color ratio with mid-infrared surface brightne ss. We find that I-v(6.75 mu m)/I-v(15 mu m) less than or similar to 1 only occurs for regions of intense heating activity where the continuum rises a t 15 mu m and where polycyclic aromatic hydrocarbon destruction can play an important role. The shape of the color-surface brightness trend also appea rs to depend, to the second order, on the hardness of the ionizing radiatio n. We discuss these findings in the context of a two-component model for th e phases of the interstellar medium and suggest that star formation intensi ty is largely responsible for the mid-infrared surface brightness and color s within normal galaxies, whereas differences in dust column density are th e primary drivers of variations in the mid-infrared surface brightness betw een the disks of normal galaxies.