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 Iν(6.75 μm)/Iν(15 μm) ratios in the inner region. We also present infrared flux densities and mid-infrared sizes for these galaxies. The mid-infrared color Iν(6.75 μm)/Iν(15 μm) shows a distinct trend with the far-infrared color Iν(60 μm)/Iν(100 μm). The quiescent galaxies in our sample [Iν(60 μm)/Iν(100 μm) ≲ 0.6] show Iν(6.75 μm)/Iν(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.