PAHs as a tracer of star formation?

Abstract

IR emission bands at 3.3, 6.2, 7.7, 8.6 and 11.3 um are generally attributed to IR fluorescence from (mainly) FUV pumped PAHs. As such, they trace the FUV stellar flux and are a measure of star formation. We examined the IR spectral characteristics of Galactic star forming regions, normal and starburst galaxies, AGNs and ULIRGs. The goal is to analyze if PAH bands are a good qualitative and/or quantitative tracer of star formation and hence the application of PAH bands as a diagnostic in order to identify the dominant processes contributing to the IR emission from Seyfert's and ULIRGs. We develop a MIR/FIR diagnostic and compare it to known diagnostics, with these also applied to the Galactic sample. This diagnostic is based on the FIR normalized 6.2 um PAH flux and the FIR normalized 6.2 um continuum flux. The Galactic sources form a sequence spanning a range of 3 orders of magnitude, from embedded compact HII regions to exposed PDRs and the (D)ISM. The variation in the 6.2 um PAH/continuum ratio is relative small. Normal and starburst galaxies ressemble exposed PDRs. While Seyfert-2's coincide with the starburst trend, Seyfert-1's are displaced by at least a factor 10 in 6.2 um continuum flux. ULIRGs show a diverse spectral appearance (AGN hot dust continuum, starburst-like or strong dust obscuration in the nucleus). ULIRGs also seems to have more prominent FIR emission than either starburst galaxies or AGNs. We discuss the observed variation in the Galactic sample in view of the evolutionary state and the PAH/dust abundance and the use of PAHs as quantitative tracers of star formation activity. We find that PAHs may be better suited as a tracer of B stars, which dominate the Galactic stellar energy budget, than as a tracer of massive star formation (O stars).