Warm dust and aromatic bands as quantitative probes of star-formation activity

We combine samples of spiral galaxies and starburst systems observed with ISOCAM on board ISO to investigate the reliability of mid-infrared dust emission as a quantitative tracer of star formation activity. The total sample covers very diverse galactic environments and probes a much wider dynamic range in star formation rate density than previous similar studies. We find that both the monochromatic 15 mum continuum and the 5-8.5 mum emission constitute excellent indicators of the star formation rate as quantified by the Lyman continuum luminosity L-Lyc, within specified validity limits which are different for the two tracers. Normalized to projected surface area, the 15 mum continuum luminosity Sigma(15mum.ct) is directly proportional to Sigma(Lyc) over several orders of magnitude. Two regimes are distinguished from the relative offsets in the observed relationship: the proportionality factor increases by a factor of approximate to5 between quiescent disks in spiral galaxies, and moderate to extreme star-forming environments in circumnuclear regions of spirals and in starburst systems. The transition occurs near Sigma(Lyc) similar to 10(2) L. pc(-2) and is interpreted as due to very small dust grains starting to dominate the emission at 15 mum over aromatic species above this threshold. The 5-8.5 mum luminosity per unit projected area is also directly proportional to the Lyman continuum luminosity, with a single conversion factor from the most quiescent objects included in the sample up to Sigma(Lyc) similar to 10(4) L. pc(-2), where the relationship then flattens. The turnover is attributed to depletion of aromatic band carriers in the harsher conditions prevailing in extreme starburst environments. The observed relationships provide empirical calibrations useful for estimating star formation rates from mid-infrared observations, much less affected by extinction than optical and near-infrared tracers in deeply embedded H II regions and obscured starbursts, as well as for theoretical predictions from evolutionary synthesis models.