E. Peeters

Polycyclic Aromatic Hydrocarbons as a Tracer of Star Formation

Infrared (IR) emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 ?m are generally attributed to IR fluorescence from (mainly) far-ultraviolet (FUV) pumped large polycyclic aromatic hydrocarbon (PAH) molecules. As such, these features trace the FUV stellar flux and are thus a measure of star formation. We examined the IR spectral characteristics of Galactic massive star-forming regions and of normal and starburst galaxies, as well as active galactic nuclei (AGNs) and ultraluminous infrared galaxies (ULIRGs). The goal of this study is to analyze whether PAH features are a good qualitative and/or quantitative tracer of star formation, and hence to evaluate the application of PAH emission as a diagnostic tool in order to identify the dominant processes contributing to the infrared emission from Seyfert galaxies and ULIRGs. We develop a new mid-infrared (MIR)/far-infrared (FIR) diagnostic diagram based on our Galactic sample and compare it to the diagnostic tools of Genzel and coworkers and Laurent and coworkers, with these diagnostic tools also applied to our Galactic sample. This MIR/FIR diagnostic is derived from the FIR normalized 6.2 ?m PAH flux and the FIR normalized 6.2 ?m continuum flux. Within this diagram, the Galactic sources form a sequence spanning a range of 3 orders of magnitude in these ratios, ranging from embedded compact H II regions to exposed photodissociation regions (PDRs) and the (diffuse) interstellar medium (ISM). However, the variation in the 6.2 ?m PAH feature-to-continuum ratio is relative small. Comparison of our extragalactic sample with our Galactic sources revealed an excellent resemblance of normal and starburst galaxies to exposed PDRs. While Seyfert 2 galaxies coincide with the starburst trend, Seyfert 1 galaxies are displaced by at least a factor of 10 in 6.2 ?m continuum flux, in accordance with general orientation-dependent unification schemes for AGNs. ULIRGs show a diverse spectral appearance. Some show a typical AGN hot dust continuum. More, however, either are starburst-like or show signs of strong dust obscuration in the nucleus. One characteristic of the ULIRGs also seems to be the presence of 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 discuss the use of PAHs as quantitative tracers of star formation activity. Based on these investigations, 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).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 Seyferts 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-2s coincide with the starburst trend, Seyfert-1s 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).

The profiles of the 3-12 micron polycyclic aromatic hydrocarbon features

We present spectra of the 3.3

FIRE AND ICE: SPITZER INFRARED SPECTROGRAPH (IRS) MID-INFRARED SPECTROSCOPY OF IRAS F00183-7111

mu

Polycyclic Aromatic Hydrocarbon Emission in the 15-21 Micron Region

m and 11.2

Deuterated interstellar polycyclic aromatic hydrocarbons

mu

High spatial resolution mid-infrared spectroscopy of the starburst galaxies NGC 3256, II Zw 40 and Henize 2-10

m PAH features of a large number of (extra-) galactic sources, obtained with ISO-SWS. Clear variations are present in the profiles of these features. The sources are classified independently based on the 3.3 and 11.2

The Prominent Dust Emission Feature near 8.9 μm in Four H II Regions

mu

Spitzer Detections of New Dust Components in the Outflow of the Red Rectangle

m feature profiles and peak positions. Correlations between these classes and those based on the 6--9

High Excitation ISM and Gas

mu

Spectroscopy of interstellar PAHs

m features (Peeters et al. 2002) are found. Also, these classifications depend on the type of object. The observed pronounced contrast in the spectral variations for the CH modes (3.3 and 11.2