The relationship between star formation rates and mid-infrared emission in galactic disks

The H alpha and mid-infrared mean disk surface brightnesses are compared in a sample of nearby spirals observed by ISOCAM. This shows that, in spiral disks, dust emission at 7 and 15 mum provides a reasonable star formation tracer. The fact that the 15 to 7 mum flux ratio is nearly constant in various global exciting conditions indicates a common origin, namely the aromatic infrared band carriers, and implies that at these wavelengths, dust emission from the disks of normal galaxies is dominated by photodissociation regions and not by HII regions themselves. We use this newly-found correlation between the mid-infrared and the H alpha line to investigate the nature of the link between the far-infrared (60 and 100 mum) and H alpha. Although the separation of the central regions from the disk is impossible to achieve in the far-infrared, we show that a circumnuclear contribution to the dust emission, having no equivalent counterpart in H alpha, is most likely responsible for the well-known nonlinearity between far-infrared and H alpha fluxes in spiral galaxies. We derive a calibration of 7 and 15 mum fluxes in terms of star formation rates from a primary calibration of H alpha in the literature, and also outline the applicability limits of the proposed conversion, which should not be blindly extrapolated to objects whose nature is unknown.