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

Abstract

The Hα 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 μm provides a reasonable star formation tracer. The fact that the 15 to 7 μm 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 H<ns0:sc>ii</ns0:sc> regions themselves. We use this newly-found correlation between the mid-infrared and the Hα line to investigate the nature of the link between the far-infrared (60 and 100 μm) and Hα. 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 Hα, is most likely responsible for the well-known non-linearity between far-infrared and Hα fluxes in spiral galaxies. We derive a calibration of 7 and 15 μm fluxes in terms of star formation rates from a primary calibration of Hα 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.