The impact of bars on the mid-infrared dust emission of spiral galaxies: global and circumnuclear properties

We study the mid-infrared properties of a sample of 69 nearby spiral galaxi es, selected to avoid Seyfert activity contributing a significant fraction of the central energetics, or strong tidal interaction, and to have normal infrared luminosities. These observations were obtained with ISOCAM, which provides an angular resolution of the order of 10" (half-power diameter of the point spread function) and low-resolution spectro-imaging information. Between 5 and 18 mum, we mainly observe two dust phases, aromatic infrared bands and very small grains, both out of thermal equilibrium. On this sampl e, we show that the global F-15/F-7 colors of galaxies are very uniform, th e only increase being found in early-type strongly barred galaxies, consist ent with previous IRAS studies. The F-15/F-7 excesses are unambiguously due to galactic central regions where bar-induced starbursts occur. However, t he existence of strongly barred early-type galaxies with normal circumnucle ar colors indicates that the relationship between a distortion of the gravi tational potential and a central starburst is not straightforward. As the p hysical processes at work in central regions are in principle identical in barred and unbarred galaxies, and since this is where the mid-infrared acti vity is mainly located, we investigate the mid-infrared circumnuclear prope rties of all the galaxies in our sample. We show how surface brightnesses a nd colors are related to both the available molecular gas content and the m ean age of stellar populations contributing to dust heating. Therefore, the star formation history in galactic central regions can be constrained by t heir position in a color-surface brightness mid-infrared diagram.