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 mid-infrared, except the prominent optical bulges are absent at 6.75 and 15 μm. In addition, the maps are quite similar at 6.75 and 15 μm, except for a few cases where a central starburst leads to lower I_v(6.75 μm)/I_v(15 μm) ratios in the inner region. We also present infrared flux densities and mid-infrared sizes for these galaxies. The mid-infrared color I_v(6.75 μm)/I_v(15 μm) shows a distinct trend with the far-infrared color I_v(60 μm)/I_v(100 μm). The quiescent galaxies in our sample [I_v(60 μm)/I_v(100 μm) ≲ 0.6] show I_v(6.75 μm)/I_v(15 μm) 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 mid-infrared 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 Å near the optical outskirts of the galaxies is approximately 8 (7) times that at 6.75 μm (15 μm), comparable to observations of the diffuse quiescent regions of the Milky Way.