H. Roussel 1, L. Vigroux 1, M. Sauvage 1, A. Bosma 2, & C. Bonoli 3
1 DSM/DAPNIA/SAp, CEA-Saclay, 91191 Gif sur Yvette Cedex FRANCE
2 Observatoire de Marseille, France
3 Osservatorio di Padova, Italy
We have gathered a sample of about forty nearby and normal spiral galaxies, half of which are barred and the others, intended to serve as a reference sample, are either non barred or of type SAB (intermediate between the two latter categories, featuring a weak bar). All these objects were observed by ISOCAM in the two broad mid-infrared filters LW2 (5 to 8.5 microns) and LW3 (12 to 18 microns) and 6 of them were also observed with the CVF imaging spectrometer (5 - 15 microns). As many of the barred galaxies are close to us and seen with modest inclinations, they are very well resolved and allow to discriminate between the emission arising from the arms, the diffuse regions, the bar and the central condensation.
One of the main focuses of the work presented here is to investigate the effect of a bar on star formation in the central regions, as it is generally believed that orbit perturbations induced by the bar potential cause the gas to flow inwards, reaching the nucleus or an inner resonance where it provides fresh material for enhanced star formation. We have found that, for barred galaxies, the mid-infrared colour LW3/LW2 is correlated with morphological type ; more precisely, the earlier the type of a spiral galaxy, the higher ratio it can reach, which tends to indicate a higher dust heating efficiency. This could be due to either a genuine stronger star formation activity or a purely geometrical effect, early galaxies having a high bulge to disk ratio and the interstellar radiation density being greater in the bulge because of an increased stellar population density. We have tested these possibilities, using indicators such as IRAS colours. We also evidence a correlation between the compactness of the mid-infrared emission and the IRAS colour F25/F12 and we confirm an excess of emission from very small grains for a fraction of barred galaxies, but not for their non-barred counterparts.