Polycyclic Aromatic Hydrocarbons as a Tracer of Star Formation?

Abstract

Infrared (IR) emission features at 3. 3, 6.2, 7.7, 8.6 and 11.3 µm are generally attributed to IR fluorescence from (mainly) 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 AGNs and ULIRGs. The goal of this study is to analyze if PAH features are a good qualitative and/or quantitative tracer of star formation and hence the application of PAH emission as a diagnostic tool in order to identify the dominant processes contributing to the infrared emission from Seyfert's and ULIRGs. We develop a new MIR/FIR diagnostic diagram based upon our Galactic sample and compare it to the diagnostic tools of Genzel et al. (1998) and(Laurent et al. 2000), with these diagnostic tools also applied to our Galactic sample. This MIR/FIR diagnostic is derived from the FIR normalized 6.2 µm PAH flux and the FIR normalized 6.2 µm 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 Photo Dissociation Regions (PDRs) and the (diffuse) ISM. However, the variation in the 6.2 µm PAH feature-to-continuum ratio is relative small. Comparison -2 -of our extragalactic sample with our Galactic sources revealed an excellent resemblance of normal and starburst galaxies to 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 µm 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, are either starburstlike 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).

THE 3.3 μm POLYCYCLIC AROMATIC HYDROCARBON EMISSION AS A STAR FORMATION RATE INDICATOR

“…It gives log ( (Figure 4). Similar trends are reported by Peeters et al (2004) and Imanishi et al (2007). In order to check if this is true, we obtain a linear fit to the detections with SB SEDs which are not ULIRGs.…”

“…Recently, Yamada et al (2012) have found that the correlation between L PAH3.3 / L IR and L IR has a break around L IR ∼ 10 12 L from another MP of AKARI, MSAGN. There is another candidate other than AGN activity that may contribute to L IR for ULIRGs: embedded young stellar objects (YSOs; Peeters et al 2004;Spoon et al 2002Spoon et al , 2007. Although embedded YSOs are from star-forming regions, they should be considered as a non-typical star-forming contribution to L IR .…”

The nuclear and extended mid-infrared emission of Seyfert galaxies

“…The PAH features are often used as indicators of the star formation rate (SFR) of galaxies (see e.g. Peeters, Spoon, & Tielens 2004;Wu et al 2005;Diamond-Stanic & Rieke 2012;Esquej et al 2014), together with low ionization potential (IP) MIR emission lines such as [Ne II]λ12.81 µm (Spinoglio & Malkan 1992;Ho & Keto 2007;Pereira-Santaella et al 2010;Spinoglio et al 2012).…”

Early Results from the Infrared Spectrograph on the Spitzer Space Telescope

“…Mid-IR spectra of Active Galactic Nuclei (AGN) often display ionic lines and also lack PAH emission features, but the continua are typically even flatter between 5-20 µm than SBS 0335-052 (see ISO observations of Seyfert 1s by Clavel et al 2000, or the nuclear spectrum of NGC 1068 by Sturm et al 2000 andLe Floc'h et al 2001 as well as Peeters et al 2004). If metal poor galaxies such as SBS 0335-052 can be present at z ∼ 0.06, it is likely that similar systems may exist not only as high-z primordial galaxies, but also at intermediate redshifts 0.5 < z < 1.5 where moderate luminosity galaxies (L IR < 10 11 L ) are accessible with the deep infrared surveys performed by Spitzer.…”