Dust in the wheel: The Cartwheel galaxy in the Mid-IR

We present mid-infrared images at 6.7 mu m and 15 mu m of "The Cartwheel" (AM 0035-33), the prototypical collisional ring galaxy. The observations, taken with ISOCAM, reveal the distribution of hot dust in the galaxy and its two companions in the north-east. The intensity of the Mid-IR emission from the outer star forming ring of the Cartwheel shows considerable azimuthal variation and peaks at the most active Ha: region of the ring. The 15 mu m to 6.7 mu m flux ratio of 5.2 is the highest among all the galaxies of our sample. A surprising result of our observations is the discovery of significant emission from the inner regions of the galaxy, including the inner ring, spokes and nucleus, where previously only low level Her emission had been reported. At 6.7 mu m, this emission is stronger than the one from the outer star forming ring, and at 15 mu m, it represents 40% of the emission from the outer ring. The Ha to Mid-IR flux ratios from the inner regions are consistent with the heating of grains from weak star formation activity.