Mid–IR colors and star formation in Virgo and Coma galaxies

We present observations of the global emission of a sample of spiral and irregular galaxies in the Virgo cluster and theComa supercluster. These observations have beenmadewith the camera ISOCAM on board the Infrared Space Observatory, through two broad filters centered respectively at 6.75 and 15.0 μm. The corresponding fluxes are corrected for the stellar contribution in order to obtain the pure interstellar contribution; the stellar contribution is estimated from the near–infrared fluxes using as template some S0 and elliptical galaxies observed by chance in the ISO fields. The fluxes at 6.75 and 15.0 μm are then normalized to the stellar mass of the galaxy, and compared to the normalized far–UV fluxes at 2000 Å which are good indicators of the 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 15μm. It appears that the carriers of theUnidentified Infrared Bands which are responsible for almost all the emission at 6.75 μm and for a part of the emission at 15 μm have different properties in these galaxies or are partly destructed by the UV field. At 15 μm, there is a contribution of very small, three–dimensional grains in galaxies with a high SFR.