MID-IR COLORS AND STAR-FORMATION IN VIRGO AND COMA GALAXIES

We present observations of the global emission of a sample of spiral a nd irregular galaxies in the Vir,oo cluster and the Coma supercluster. These observations have been made with the camera ISOCAM on board the Infrared Space Observatory, through two broad filters centered respec tively at 6.75 and 15.0 mu m. The corresponding fluxes are corrected f or the stellar contribution in order to obtain the pure interstellar c ontribution; the stellar contribution is estimated from the near-infra red fluxes using as template some SO and elliptical galaxies observed by chance in the ISO fields. The fluxes at 6.75 and 15.0 mu m are then normalized to the stellar mass of the galaxy, and compared to the nor malized far-UV fluxes at 2000 Angstrom which are good indicators of th e star-formation rate (SFR). We find that, while the mid-IR fluxes are proportional to the SFR when it is not too large, the galaxies with a high SFR have relatively less emission at 6.75 and 15 mu m. It appear s that the carriers of the Unidentified Infrared Bands which are respo nsible for almost all the emission at 6.75 mu m and for a part of the emission at 15 mu m have different properties in these galaxies or are partly destructed by the UV field. At 15 mu m, there is a contributio n of very small, three-dimensional grains in galaxies with a high SFR.