Strong IR emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 μm are a common characteristic of the interstellar medium of the Milky Way and nearby galaxies and out to redshifts of3. Here, we review the history of the PAH hypothesis which attributes these emission features to vibrational fluorescence of large (~50 carbon-atom Polycyclic Aromatic Hydrocarbon molecules pumped by ultraviolet photons from nearby stars or the average interstellar radiation field. Over the last 25 years, our insight in the characteristics of these molecules and their role in the Universe has greatly improved and the PAH hypothesis is alive and well; not in the least due a remarkable adaptability. Not surprisingly, the precise characteristics of these species remains to be defined.

The infrared (IR) spectra of many galactic and extragalactic objects are dominated by emission features at 3.3, 6.2, 7.7, 8.6 and 11.2 μm, generally attributed to the IR fluorescence of Polycyclic Aromatic Hydrocarbon molecules (PAHs). These PAH bands have been found in a wide variety of environments throughout theUniverse and contain up to 10% of the total power output of star-forming galaxies.

Ground-based telescopes, the Infrared Space Observatory (ISO) and the Spitzer Space Telescope revealed a plethora of weaker PAH bands and provided extensive evidence for significant variability in the PAH spectrum from source to source and spatially within sources. Here we review the spectral characteristics of astronomical PAHs, their dependence on the local physical conditions and the implications for the physical and chemical characteristics of the carriers.

Physical dust models which include a PAH component are quite successful in reproducing the measured extinction vs. wavelength in the Milky Way, the infrared (IR) emission observed from Milky Way regions and other galaxies, and a number of other observational constraints. Many of the adopted PAH properties are necessarily highly idealized.

The observed variations in the 7.7 μm/11.3 μm band ratio can be reproduced by varying the PAH ionization balance. Changing the spectrum of the starlight heating the PAHs affects the overall strength of the PAH emission (relative to total IR), but has relatively little effect on the 7.7 μm/11.3 μm band ratio.

This paper reviews the various processes that have been proposed to explain the observed trend of PAH strength with metallicity. It summarizes a study showing that, although PAH destruction by hard radiation is important in low-metallicity environments, it is not sufficient to explain their paucity. In these systems, the deficiency of their formation mechanism, related to stellar evolution, has to be invoked.

A large number of galactic and extragalactic sources exhibit a mid-IR spectrum that is dominated by PAH emission. Looking at these spectra in more details reveals a strong variability in the shape/position of the observed features, depending on the observed source, or even the region within a source. In this article, we present the results of an analysis that has allowed us to decompose these spectra into components having a physical meaning. Most, if not all PAH dominated mid-IR spectra of HII regions, PDRs, protoplanetary disks, galaxies etc. can be fitted efficiently using a combination of these components. The results of these fits provide further insight in the composition of the emitting material and the local physical conditions. In the frame of the future IR space missions (JWST, SPICA), this approach can be very useful to probe the physical conditions in distant galaxies.

Near-infrared (NIR; 2.5–5 μm) low-resolution (λ/Δλ ~ 100) spectra were obtained for a number of Galactic and extragalactic objects with the Infrared Camera (IRC) in the AKARI warm mission. These data provide us with the first opportunity to make a systematic study of the 3.3–3.5 μm PAH features in a galactic scale as well as within an object. Whereas the 3.3 μm band is well resolved in most spectra, the 3.5 μm band is not clearly separated from the 3.4 μm band in the IRC spectrum. The intensity ratio of the summation of the 3.4 and 3.5 μm bands to the 3.3 μm band shows a tendency to increase towards the Galactic center, although a large variation in the ratio is also seen in a local scale. A search for deuterated PAH features in the 4 μm region is carried out in IRC NIR spectra. Emission lines originating from the ionized gas together with the detector anomaly hamper an accurate search at certain wavelengths, but little convincing evidence has so far been obtained for the presence of significant features in 4.2–4.7 μm. A conservative upper limit of a few percents is obtained for the integrated intensity ratio of the 4.4–4.7 μm possible features to the 3.3–3.5 μm PAH features in the spectra so far obtained.

The hypothesis that polyaromatic molecules are the carriers of the infrared interstellar emission bands has spurred the laboratory spectroscopy of this class of molecules. Here we will give an overview of the infrared spectroscopic methods that have been applied over the past two decades to investigate the IR spectra of PAHs, their ions and related species.

In the long story of interstellar PAHs, computations have played and are still playing a fundamental role in connection with experiments and observations. From the very first calculations of the IR spectra of small PAHs in the late eighties to the more recent ones, every aspect of the research linked to the PAH hypothesis has evolved dramatically: the size and the variety of the molecules considered, the techniques used, the precision of the astronomical observations ... The initial landscape has completely changed though the quest is still the same, that is to correlate the so-called UIR bands spectra ubiquitous in the ISM (Inter Stellar Medium) with a chemical family of molecules, the PAHs. An historical review of the 25 years of this quest is presented here, focusing on the computational part.

The IR emission cascade from the pyrene cation due to a broad band optical excitation is simulated using kinetic Monte Carlo. Anharmonicities of the ground electronic state potential energy surface are taken into account in the transition energies, the microcanonical densities of states, and the rate of hydrogen loss through various statistical theories. The emission spectral features of the “3.3”, “6.2” and “11.2” μm bands are computed for different blackbody temperatures.

In this contribution, we summarize the recent progress made in recording laboratory infrared (IR) spectra of protonated polycyclic aromatic hydrocarbon molecules (H+PAH) in the gas phase. The IR spectra of a large variety of H+PAH species ranging from benzene to coronene have been obtained by various variants of photodissociation spectroscopy. The employed techniques include single-photon IR photodissociation (IRPD) of tagged H+PAH ions and IR multiple-photon dissociation (IRMPD) of bare H+PAH ions. The comparison of the laboratory IR spectra with astronomical spectra supports the hypothesis that H+PAH ions are possible carriers of the unidentified IR emission (UIR) bands. Moreover, the spectra provide detailed information about the geometric and electronic structure as well as the chemical reactivity and stability of these fundamental hydrocarbon ions.

Polycyclic Aromatic Hydrocarbons (PAHs) are widespread across the Universe and influence many stages of the Galactic lifecycle. The presence of PAHs has been well established and the rich mid-IR PAH spectrum is now commonly used as a probe into (inter)stellar environments. The NASA Ames PAH IR Spectroscopic Database has been key to test and refine the “PAH hypothesis”. This database is a large coherent set (>600 spectra) of laboratory measured and DFT computed infrared spectra of PAHs from C10H8 to C130H28 and has been made available on the web at (http://www.astrochem.org/pahdb). With a new spectral window opening up; the far-IR, the study of PAH far-IR spectra and the quest for identifying a unique member of the interstellar PAH family has begun. To guide this research, the far-IR (>20 μm) spectra of different sets of PAHs are investigated using the NASA Ames PAH IR Spectroscopic Database. These sets explore the influence of size, shape, charge and composition on the far-IR PAH spectrum. The far-IR is also the domain of the so-called “drumhead” modes and other molecular vibrations involving low order bending vibrations of the carbon skeleton as a whole. As with drums, these are molecule and shape specific and promise to be a key diagnostic for specific PAHs. Here, the sensitivity of these “drumhead” modes to size and shape is assessed by comparing the frequencies of the lowest drumhead modes of a family of circular shaped (the coronene “family”) and rhombus shaped (the pyrene “family”) PAH molecules. From this study, some consequences for an observing strategy are drawn.

We use the NASA Ames Polycyclic Aromatic Hydrocarbon (PAH) infrared spectroscopic database to model infared emission of PAHs following absorption of a UV photon. We calculate emission spectra resulting from the full cooling cascade for each species in the database. Using a least squares approach, we can find out what PAH mixtures best reproduce a few typical astronomical observations representing the different classes of UIR spectra. We find that we can reproduce the observed UIR spectra in the wavelength range 6–14 μm, offering support for the hypothesis that the UIR bands are indeed due to vibrational modes of PAHs and related molecular species. Spectral decompositions of our best fit models confirm and reinforce several earlier results: (i) the 6.2 μm band requires a significant contribution of nitrogen-substituted PAHs (PANHs); (ii) the reported components and their variations in the 7.7 μm band are indicative of changes in the size distribution of the contributing molecules; (iii) there is a significant contribution of anions to the 7.7 μm band; (iv) the 11.2 μm band is due to large, neutral and pure PAHs; (v) the 11.0 μm band is due to large PAH cations.

Herschel opens the possibility to detect the low-frequency vibrational bands of individual polycyclic aromatic hydrocarbon (PAH) molecules and therefore to progress in our understanding of the nature of these species and the properties of the environments from which they emit. However, unless one individual molecule dominates the PAH family, this detection will not be straightforward and it is necessary to optimise the observational search with an educated guess of the band profiles and intensities. Such educated guess can be obtained from models that include a detailed description of the molecular properties (anharmonicity, rotation...) in the modelling of the cooling cascade of the emitting species. First results are expected soon from the observation of the Orion Bar as part of the HEXOS Herschel key program.

As images and spectra from ISO and Spitzer have provided increasingly higher–fidelity representations of the mid–infrared (MIR) and Polycyclic Aromatic Hydrocarbon (PAH) emission from galaxies and galactic and extra–galactic regions, more systematic efforts have been devoted to establishing whether the emission in this wavelength region can be used as a reliable star formation rate indicator. This has also been in response to the extensive surveys of distant galaxies that have accumulated during the cold phase of the Spitzer Space Telescope. Results so far have been somewhat contradictory, reflecting the complex nature of the PAHs and of the mid–infrared–emitting dust in general. The two main problems faced when attempting to define a star formation rate indicator based on the mid–infrared emission from galaxies and star–forming regions are: (1) the strong dependence of the PAH emission on metallicity; (2) the heating of the PAH dust by evolved stellar populations unrelated to the current star formation. I review the status of the field, with a specific focus on these two problems, and will try to quantify the impact of each on calibrations of the mid–infrared emission as a star formation rate indicator.

We characterize PAH populations in 22 metal-poor blue compact dwarf galaxies (BCDs), 16 of which have an oxygen abundance 12+log(O/H) ≲ 8. This is the largest sample ever studied at such low metallicities. The relative PAH intensities of the 6.2, 7.7, 8.6 and 11.3 μm features in these BCDs suggest a deficit of small PAH carriers, or alternatively, an excess of large ones at these low abundances.

With advancement of infrared space telescopes during the past decade, infrared wavelength regime has been a focal point to study various properties of galaxies with respect to evolution of galaxies. Polycyclic Aromatic Hydrocarbons (PAHs) have emerged as one of the most important features since these features dominate the mid-infrared spectra of galaxies. These PAH features provide a great handle to calibrate star formation rates and diagnose ionized states of grains. However, the PAH 3.3 μm feature has not been studied as much as other PAH features since it is weaker than others and resides outside of Spitzer capability, although it will be the only PAH feature accessible by JWST for high-z galaxies. AKARI mJy Unbiased Survey of Extragalactic Sources in 5MUSES (AMUSES) intends to take advantage of AKARI capability of spectroscopy in the 2 ~ 5   μm to provide an unbiased library of 44 sample galaxies selected from a parent sample of 5MUSES, one of Spitzer legacy projects. For these 3.6 μm flux limited sample galaxies whose redshifts range between 0 < z < 1, AMUSES will calibrate PAH 3.3 μm as a star formation rate (SFR) indicator while measuring ratios between PAH features. We present preliminary results of AMUSES.

We review observational results of PAH emission in harsh interstellar environments, which are mostly based on recent works by Spitzer and AKARI. The harsh environments include shock regions in our Galaxy, the ionized superwinds and haloes of external galaxies, and the hot plasmas of elliptical galaxies. Owing to the unprecedented high sensitivity for PAH emission with Spitzer and AKARI, it is found that an appreciable amount of PAHs are present in some cases with such hostile conditions. Some of them exhibit unusual PAH interband strength ratios, reflecting either evolution of PAHs or physical conditions of the ISM. The distribution of the PAH emission, as compared to that of dust emission, is shown to discuss their ways of evolution and survival.

I present observations of shock-processed PAHs and dust in supernova remnants (SNRs). Supernova shocks are one of the primary sites destroying, fragmenting and altering interstellar PAHs and dust. Studies of PAHs through supernova shocks had been limited because of confusion with PAHs in background emission. Spitzer observations with high sensitivity and resolution allow us to separate PAHs associated with the SNRs and unrelated, Galactic PAHs. In the young SNR N132D, PAH features are detected with a higher PAH ratio of 15–20/7.7 μm than those of other astronomical objects, and we suggest large PAHs have survived behind the shock. We present the spectra of additional 14 SNRs observed with Spitzer IRS and MIPS SED covering the range of 5–90 μm. Bright PAH features from 6.2 to 15–20 μm are detected from many of SNRs which emit molecular hydrogen lines, indicating that both large and small PAHs survive in low velocity shocks. We observe a strong correlation between PAH detection and carbonaceous small grains, while a few SNRs with dominant silicate dust lack PAH features. We characterize PAHs depending on the shock velocity, preshock density and temperature of hot gas, and discuss PAH and dust processing in shocks and implication of PAH and dust cycles in ISM.

The formation of Polycyclic Aromatic Hydrocarbons in the circumstellar outflows of evolved stars is reviewed, with an emphasis on carbon stars on the Asymptotic Giant Branch. Evidence for PAHs present in their winds is provided by meteoritic studies and recent observations of the Unidentified Infrared bands. We detail the chemical processes leading to the closure of the first aromatic ring as well as the growth mechanisms leading to amorphous carbon grains. Existing studies on PAH formation in evolved stellar envelopes are reviewed and new results for the modelling of the inner wind of the archetype carbon star IRC+10216 are presented. Benzene, C6H6, forms close to the star, as well as water, H2O, as a result of non-equilibrium chemistry induced by the periodic passage of shocks. The growth process of aromatic rings may thus resemble that active in sooting flames due to the presence of radicals like hydroxyl, OH. Finally, we discuss possible formation processes for PAHs and aromatic compounds in the hydrogen-rich R CrB star, V854 Cen, and their implication for the carriers of the Red Emission and the Diffuse Interstellar Bands.