ISO mid-infrared observations of normal star-forming galaxies: The key project sample

We present mid-infrared maps and preliminary analysis for 61 galaxies observed with the ISOCAM instrument aboard the Infrared Space Observatory. Many of the general features of galaxies observed at optical wavelengths-spiral arms, disks, rings, and bright knots of emission-are also seen in the midinfrared, except the prominent optical bulges are absent at 6.75 and 15 mu m In addition, the maps are quite similar at 6.75 and 15 mu m, except for a few cases where a central starburst leads to lower I-nu(6.75 mu m)/I-nu(15 mu m) ratios in the inner region. We also present infrared flux densities and mid-infrared sizes for these galaxies. The mid-infrared color I-nu(6.75 mu m)/I-nu(15 mu m) shows a distinct trend with the far-infrared color I-nu(60 mu m)/I-nu(100 mu m). The quiescent galaxies in our sample [I-nu(60 mu m)/I-nu(100 mu m) less than or similar to 0.6] show I-nu(6.75 mu m)/I-nu(15 mu m) near unity, whereas this ratio drops significantly for galaxies with higher global heating intensity levels. Azimuthally averaged surface brightness profiles indicate the extent to which the mid-infrared flux is centrally concentrated, and provide information on the radial dependence of midinfrared colors. The galaxies are mostly well resolved in these maps: almost half of them have < 10% of their flux in the central resolution element. A comparison of optical and mid-infrared isophotal profiles indicates that the flux at 4400 Angstrom near the optical outskirts of the galaxies is approximately 8 (7) times that at 6.75 mu m (15 mu m), comparable to observations of the diffuse quiescent regions of the Milky Way.