8 Summary and conclusions

We have studied the mid-infrared activity induced by bars in a sample of 69 nearby spiral galaxies with infrared luminosities spanning a large range below the class of luminous infrared galaxies. We have found that:

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The mid-infrared emission of the normal galaxies in our sample is essentially contributed by a thermal continuum from very small grains (VSGs) longward of 10$\mu$m and the family of aromatic bands (UIBs) detected in a wide diversity of environments. It is the variation of the VSG component with respect to the UIBs that is responsible for F₁₅/F₇ changes in our galaxies. From the comparison with observations of resolved Galactic regions, this can be related to an increase of the filling factor of star forming complexes by photoionized regions, hence a decrease of the contribution to the mid-infrared emission from neutral and molecular media.

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There is a dichotomy between spiral disks, where the integrated F₁₅/F₇ color is close to 1 and shows little dispersion, and circumnuclear regions, where F₁₅/F₇ ranges from disk-like to high values (up to 4). We have found no indication that destruction of UIB carriers occurs at the scale of circumnuclear regions, although it would be desirable to analyze infrared spectroscopic data of the most active galaxies, to elaborate on this.

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We confirm that barred spirals distinguish themselves from unbarred galaxies in the sense that they can reach higher F₁₅/F₇ colors. This effect is however restricted to early morphological types, in agreement with previous IRAS-based studies (Hawarden et al. 1986; Huang et al. 1996). We show unambiguously that this emission excess arises in circumnuclear regions which can completely dominate the mid-infrared emission, although their size remains modest ( $D_{\, \rm CNR}$ ranges between 2 and 26% of the optical diameter in the whole sample, with no clear dependency on Hubble type). Galaxies with a global color excess are all dominated by their central regions. This is a confirmation of predictions from hydrodynamical models (Athanassoula 1992; Friedli & Benz 1993), according to which a barred perturbation, through tidal torques and shocks, induces substantial mass transfer towards circumnuclear regions. We observe the consequences of these gas flows, i.e. the intense star formation that they fuel.

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An important fact to mention is that only a fraction of early-type barred galaxies can be distinguished from unbarred galaxies in their infrared properties. Several interrelated parameters may explain this quiescence of many barred galaxies. With the present data we are unable to address this issue and thus only list possible explanations: a bar evolves on a much longer timescale than a starburst (Martinet & Friedli 1997) and the accretion rate by a bar is slow; the inward mass transfer is regulated by the depth of the potential well, the intensity of the shocks inside the bar, the star formation efficiency along the path of the inflowing gas before it reaches central regions (Martin & Friedli 1997), etc.

Although the presence of a bar can be an efficient means of triggering circumnuclear starbursts, the dust emission processes in central regions are the same in barred and unbarred galaxies. We have studied the properties of these central regions at the degree of detail accessible to our spatial resolution. Several physical properties were found to control the mid-infrared color F₁₅/F₇.

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The estimated molecular gas content inside the central regions ( $R_{\rm CNR}$): as expected, the mid-infrared brightness at 7$\mu$m tends to rise with increasing mean gas density, which reflects the physical association of dust with molecular material. The F₁₅/F₇ color is also correlated with the mean gas density. As a higher gas content allows more efficient star formation on relatively large scales, according to the Schmidt law and stability criteria or other empirical laws (Kennicutt 1998), this supports an interpretation of mid-infrared colors in terms of starburst intensity. A few galaxies, showing the most extreme F₁₅/F₇ ratios, depart from the general trend. This could be interpreted as a transient evolutionary state of the starburst during which most of the gas has been consumed or dispersed, although a definite assessment of the effect requires better molecular line data.

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The age of the stellar populations heating dust: dust is sensitive to star formation on relatively long timescales, as can be expected from the fact that it can be heated by optical and near-UV photons. However, the strength of the VSG continuum is more sensitive to the radiation energy density and hardness than UIBs, and the results that we report here indicate that F₁₅/F₇ excesses are linked to the weighted age of the exciting stellar populations. Mid-infrared colors are therefore influenced by the previous star formation history over at least 1Gyr, and depend on the fraction of the ultraviolet radiation power contributed by young populations created in a contemporary starburst, with respect to intermediate-age populations from already faded bursts.

Acknowledgements

We thank our referee, Louis Martinet, for his helpful remarks.

The ISOCAM data presented in this paper were analyzed using and adapting the CIA package, a joint development by the ESA Astrophysics Division and the ISOCAM Consortium (led by the PI C. Cesarsky, Direction des Sciences de la Matière, CEA, France).