An Introduction to Polycyclic Aromatic Hydrocarbons
Many extragalactic objects display a set of distinctive infrared emission features at wavelengths between 3 and 12 μm. The first observations to reveal these emission lines were published in 1973 by Gillett, Forrest, and Merrill. For this paper, Gillett et al. observed three Galactic planetary nebulae: NGC 7027, BD+30°3639, and NGC 6572. While the main focus of the article is on known infrared molecular lines, there is some discussion of a strong peak at 11.3 μm that was detected in both NGC 7027 and BD+30°3639. In the spectra of both of these sources, this 11.3 μm feature appears broader than the known atomic emission lines that were also measured in both nebulae. Other results published during the 1970s and 1980s showed similar peaks over a range of close wavelengths (Russell et al. 1977; Bregman et al. 1983). Since, at the time, the astrochemistry of the interstellar medium was not entirely well understood, these mysterious emission features were termed the “Unidentified Infrared” or UIR bands and their origin was left up for debate.
In 1985, Allamandola and Tielens hypothesized that the UIR emission features might originate from aromatic species (Allamandola et al. 1985). To test this theory, they obtained Raman spectra of auto exhaust and compared the emission pattern to a spectrum taken of the Orion Bar. Both of these spectra are shown below. Due to the close agreement between the two spectra, they claimed that the emission in both must arise from similar molecular species. Thus, Allamandola and Tielens concluded that the UIR features must originate from polycyclic aromatic hydrocarbon (PAH) molecules that are being excited in the interstellar medium.
The Chemistry of PAHs
Polycyclic aromatic hydrocarbons or PAHs are complex organic planar molecules (Harvey 1991). Each molecule is a few angstroms in size and typically contains on the order of 20 or more carbon atoms. The smallest PAH molecule is Naphthalene containing only ten carbon atoms (CH). Ovalene, a much larger PAH molecule has thirty-two carbon atoms (CH). Each PAH molecule is built up from fused aromatic rings containing six carbon atoms each. This basic building structure of all PAH molecules is called a “benzene ring” after the simplest such cyclic aromatic hydrocarbon. Such a molecule is defined as “aromatic” when the bonds between carbon atoms alternate between single and double. For reference, molecular diagrams of Benzene, Naphthalene, and Ovalene are shown below.
The strongest observed PAH features occur at wavelengths of 5.7, 6.2, 7.7, 8.6, and 11.3 μm. Each of these emission lines is produced when a molecule is vibrationally excited after absorbing a single ultraviolet or visible photon. Some work has been done to identify the specific vibrational and bending modes that correspond to each emission line. For example, the 11.3 μm line is thought to result from the bending of the C−H bond out of the plane of the molecule (Allamandola et al. 1985). On the other hand, the 8.6 μm line is due to the bending of the C−H bond within the plane of the molecule. In addition to bending, these bonds can also stretch and contract. The 3.3 μm line is due to such stretching in the C−H bond. By now, these distinctive lines have been observed in a wide range of astrophysical objects: post-AGB stars, planetary nebulae, HII regions, the ISM, and other extragalactic sources. Take for example, this Spitzer image looking towards the plane of the Milky Way. All of the green emission in this image comes from PAH molecules.
The Formation of PAHs
In order for PAHs to be observed in such a wide range of extragalactic sources, there needs to be some production method for them in the ISM. The most widely accepted theory is that PAHs originate from Asymptotic Giant Branch (AGB) stars (Lebouteiller et al. 2011; Galliano et al. 2008). Aliphatic or non-aromatic hydrocarbons have been detected in the circumstellar envelopes of carbon-rich AGB stars. Through photo processing of the C−H bonds in these molecules, aromatic hydrocarbons are formed. These PAHs are then ejected into the ISM by stellar winds.
This formation scenario results in several implications for the amount of PAH molecules in galaxies, as well as for the characteristics of the environments that PAH molecules
are found in. To begin with, the amount of PAH production in a galaxy should follow the evolutionary trend of AGB stars. This implies that there will be a delayed injection of PAH molecules into the ISM. A young galaxy that is experiencing its first period of star formation should not display much PAH emission, because there will not have been enough time for significant amounts of carbon dust to have been injected into the ISM by evolving low mass stars. Secondly, the amount of PAH formation should depend on the ISM metallicity. A more metal-enriched ISM implies that AGB stars will continue to contribute increased amounts of complex molecules to the chemical enrichment of the ISM. Finally, the PAH abundance should peak in sources with solar metallicity or higher. This boundary marks the transition from oxygen-rich to carbon-rich AGB stars. Thus, higher metallicity sources can be expected to have more carbon-rich AGB stars to inject PAHs into the ISM.
Of course, there are several other theories as to how PAH molecules form in the ISM. It has been suggested that PAHs might form in strong interstellar shocks from grain-grain collisions (Tielens et al. 1987). Other proposed formation methods include ion-molecule reactions in dense interstellar clouds (Herbst 1991) and the accretion of C+ ions (Omont 1986). Unfortunately, it has been impossible to place any observational constraints on these alternative formation methods to date. If one of these methods proved to be valid, it would greatly affect some of the conclusions about the effect of environment on PAH abundance.
The Connection Between PAH Emission and Star Formation
The interstellar medium obscures most direct tracers of star formation that are observable in the visible or ultraviolet bands. Since PAH molecules are vibrationally excited by visible and UV photons, emission from these molecules is expected to trace these same star formation regions. However, since PAH features are in the infrared, these emission lines should be much more easily observed in regions of star formation. Infrared emission is typically attenuated by an order of magnitude less than visible or ultraviolet emission (Hogg et al. 2005). Since infrared emission is much more revealing in star formation regions, it has been proposed that by measuring the strength of PAH emission in distant galaxies, conclusions can be drawn about the amount of star formation occurring in those galaxies.