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

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

Ground-based telescopes, the Infrared Space Observatory (ISO) and the Spitzer SpaceTelescope revealed a plethora of weaker PAH bands and provided extensive evidence forsignificant variability in the PAH spectrum from source to source and spatially withinsources. Here we review the spectral characteristics of astronomical PAHs, theirdependence on the local physical conditions and the implications for the physical andchemical characteristics of the carriers.

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

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

This paper reviews the various processes that have been proposed to explain the observedtrend of PAH strength with metallicity. It summarizes a study showing that, although PAHdestruction by hard radiation is important in low-metallicity environments, it is notsufficient to explain their paucity. In these systems, the deficiency of their formationmechanism, related to stellar evolution, has to be invoked.

A large number of galactic and extragalactic sources exhibit a mid-IR spectrum that isdominated by PAH emission. Looking at these spectra in more details reveals a strongvariability in the shape/position of the observed features, depending on the observedsource, or even the region within a source. In this article, we present the results of ananalysis that has allowed us to decompose these spectra into components having a physicalmeaning. 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 thesecomponents. The results of these fits provide further insight in the composition of theemitting material and the local physical conditions. In the frame of the future IR spacemissions (JWST, SPICA), this approach can be very useful to probe the physical conditionsin distant galaxies.

Near-infrared (NIR; 2.5–5 μm) low-resolution(λ/Δλ ~ 100) spectra were obtained for anumber of Galactic and extragalactic objects with the Infrared Camera (IRC) in the AKARIwarm mission. These data provide us with the first opportunity to make a systematic studyof the 3.3–3.5 μm PAH features in a galactic scale as well as within anobject. Whereas the 3.3 μm band is well resolved in most spectra, the3.5 μm band is not clearly separated from the 3.4 μmband in the IRC spectrum. The intensity ratio of the summation of the 3.4 and3.5 μm bands to the 3.3 μm band shows a tendency toincrease towards the Galactic center, although a large variation in the ratio is also seenin a local scale. A search for deuterated PAH features in the 4 μm regionis carried out in IRC NIR spectra. Emission lines originating from the ionized gastogether with the detector anomaly hamper an accurate search at certain wavelengths, butlittle convincing evidence has so far been obtained for the presence of significantfeatures in 4.2–4.7 μm. A conservative upper limit of a few percents isobtained for the integrated intensity ratio of the 4.4–4.7 μm possiblefeatures to the 3.3–3.5 μm PAH features in the spectra so farobtained.

The hypothesis that polyaromatic molecules are the carriers of the infrared interstellaremission bands has spurred the laboratory spectroscopy of this class of molecules. Here wewill give an overview of the infrared spectroscopic methods that have been applied overthe past two decades to investigate the IR spectra of PAHs, their ions and relatedspecies.

In the long story of interstellar PAHs, computations have played and are still playing afundamental role in connection with experiments and observations. From the very firstcalculations 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: thesize and the variety of the molecules considered, the techniques used, the precision ofthe astronomical observations ... The initial landscape has completelychanged though the quest is still the same, that is to correlate the so-called UIR bandsspectra 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, focusingon the computational part.

The IR emission cascade from the pyrene cation due to a broad band optical excitation issimulated using kinetic Monte Carlo. Anharmonicities of the ground electronic statepotential energy surface are taken into account in the transition energies, themicrocanonical densities of states, and the rate of hydrogen loss through variousstatistical 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 laboratoryinfrared (IR) spectra of protonated polycyclic aromatic hydrocarbon molecules(H+PAH) in the gas phase. The IR spectra of a large variety ofH+PAH species ranging from benzene to coronene have been obtained by variousvariants of photodissociation spectroscopy. The employed techniques include single-photonIR photodissociation (IRPD) of tagged H+PAH ions and IR multiple-photondissociation (IRMPD) of bare H+PAH ions. The comparison of the laboratory IRspectra with astronomical spectra supports the hypothesis that H+PAH ions arepossible carriers of the unidentified IR emission (UIR) bands. Moreover, the spectraprovide detailed information about the geometric and electronic structure as well as thechemical reactivity and stability of these fundamental hydrocarbon ions.

Polycyclic Aromatic Hydrocarbons (PAHs) are widespread across the Universe and influencemany stages of the Galactic lifecycle. The presence of PAHs has been well established andthe rich mid-IR PAH spectrum is now commonly used as a probe into (inter)stellarenvironments. The NASA Ames PAH IR Spectroscopic Database has been key to test and refinethe “PAH hypothesis”. This database is a large coherent set (>600 spectra) oflaboratory measured and DFT computed infrared spectra of PAHs fromC10H8 to C130H28 and has been made availableon the web at (http://www.astrochem.org/pahdb). With a new spectral window opening up; thefar-IR, the study of PAH far-IR spectra and the quest for identifying aunique member of the interstellar PAH family has begun. To guide this research, the far-IR(>20 μm) spectra of different sets of PAHs are investigatedusing the NASA Ames PAH IR Spectroscopic Database. These sets explore the influence ofsize, shape, charge and composition on the far-IR PAH spectrum. The far-IR is also thedomain of the so-called “drumhead” modes and other molecular vibrations involving loworder bending vibrations of the carbon skeleton as a whole. As with drums, these aremolecule 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 thefrequencies 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, someconsequences for an observing strategy are drawn.

We use the NASA Ames Polycyclic Aromatic Hydrocarbon (PAH) infrared spectroscopicdatabase to model infared emission of PAHs following absorption of a UV photon. Wecalculate emission spectra resulting from the full cooling cascade for each species in thedatabase. Using a least squares approach, we can find out what PAH mixtures best reproducea 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 6–14μm, offering support for the hypothesis that the UIR bands are indeeddue to vibrational modes of PAHs and related molecular species. Spectral decompositions ofour best fit models confirm and reinforce several earlier results: (i)the 6.2 μm band requires a significant contribution ofnitrogen-substituted PAHs (PANHs); (ii) the reported components and theirvariations in the 7.7 μm band are indicative of changes in the sizedistribution of the contributing molecules; (iii) there is a significantcontribution of anions to the 7.7 μm band; (iv) the 11.2μm 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 vibrationalbands of individual polycyclic aromatic hydrocarbon (PAH) molecules and therefore toprogress in our understanding of the nature of these species and the properties of theenvironments from which they emit. However, unless one individual molecule dominates thePAH family, this detection will not be straightforward and it is necessary to optimise theobservational search with an educated guess of the band profiles and intensities. Sucheducated guess can be obtained from models that include a detailed description of themolecular properties (anharmonicity, rotation...) in the modelling of the cooling cascadeof the emitting species. First results are expected soon from the observation of the OrionBar as part of the HEXOS Herschel key program.

As images and spectra from ISO and Spitzer have provided increasingly higher–fidelityrepresentations of the mid–infrared (MIR) and Polycyclic Aromatic Hydrocarbon (PAH)emission from galaxies and galactic and extra–galactic regions, more systematic effortshave been devoted to establishing whether the emission in this wavelength region can beused as a reliable star formation rate indicator. This has also been in response to theextensive surveys of distant galaxies that have accumulated during the cold phase of theSpitzer Space Telescope. Results so far have been somewhat contradictory, reflecting thecomplex nature of the PAHs and of the mid–infrared–emitting dust in general. The two mainproblems faced when attempting to define a star formation rate indicator based on themid–infrared emission from galaxies and star–forming regions are: (1) the strongdependence of the PAH emission on metallicity; (2) the heating of the PAH dust by evolvedstellar populations unrelated to the current star formation. I review the status of thefield, with a specific focus on these two problems, and will try to quantify the impact ofeach 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), 16of which have an oxygen abundance 12+log(O/H) ≲ 8. This is the largest sample ever studiedat such low metallicities. The relative PAH intensities of the 6.2, 7.7, 8.6 and11.3 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 wavelengthregime has been a focal point to study various properties of galaxies with respect toevolution of galaxies. Polycyclic Aromatic Hydrocarbons (PAHs) have emerged as one of themost important features since these features dominate the mid-infrared spectra ofgalaxies. These PAH features provide a great handle to calibrate star formation rates anddiagnose ionized states of grains. However, the PAH 3.3 μm feature hasnot been studied as much as other PAH features since it is weaker than others and residesoutside of Spitzer capability, although it will be the only PAH feature accessible by JWSTfor high-z galaxies. AKARI mJy Unbiased Survey of Extragalactic Sources in 5MUSES (AMUSES)intends to take advantage of AKARI capability of spectroscopy in the2 ~ 5   μm to provide an unbiased library of 44 samplegalaxies selected from a parent sample of 5MUSES, one of Spitzer legacy projects. Forthese 3.6 μm flux limited sample galaxies whose redshifts range between0 < z < 1, AMUSES will calibrate PAH 3.3μm as a star formation rate (SFR) indicator while measuring ratiosbetween PAH features. We present preliminary results of AMUSES.

We review observational results of PAH emission in harsh interstellar environments, whichare mostly based on recent works by Spitzer and AKARI. The harsh environments includeshock regions in our Galaxy, the ionized superwinds and haloes of external galaxies, andthe hot plasmas of elliptical galaxies. Owing to the unprecedented high sensitivity forPAH emission with Spitzer and AKARI, it is found that an appreciable amount of PAHs arepresent in some cases with such hostile conditions. Some of them exhibit unusual PAHinterband strength ratios, reflecting either evolution of PAHs or physical conditions ofthe ISM. The distribution of the PAH emission, as compared to that of dust emission, isshown 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 alteringinterstellar PAHs and dust. Studies of PAHs through supernova shocks had been limitedbecause of confusion with PAHs in background emission. Spitzerobservations with high sensitivity and resolution allow us to separate PAHsassociated with the SNRs and unrelated, Galactic PAHs. In the young SNR N132D, PAHfeatures are detected with a higher PAH ratio of 15–20/7.7 μm than thoseof other astronomical objects, and we suggest large PAHs have survived behind the shock.We present the spectra of additional 14 SNRs observed with Spitzer IRSand MIPS SED covering the range of 5–90 μm. Bright PAH features from 6.2to 15–20 μm are detected from many of SNRs which emit molecular hydrogenlines, indicating that both large and small PAHs survive in low velocity shocks. Weobserve a strong correlation between PAH detection and carbonaceous small grains, while afew SNRs with dominant silicate dust lack PAH features. We characterize PAHs depending onthe shock velocity, preshock density and temperature of hot gas, and discuss PAH and dustprocessing in shocks and implication of PAH and dust cycles in ISM.

The formation of Polycyclic Aromatic Hydrocarbons in the circumstellar outflows ofevolved stars is reviewed, with an emphasis on carbon stars on the Asymptotic GiantBranch. Evidence for PAHs present in their winds is provided by meteoritic studies andrecent observations of the Unidentified Infrared bands. We detail the chemical processesleading to the closure of the first aromatic ring as well as the growth mechanisms leadingto amorphous carbon grains. Existing studies on PAH formation in evolved stellar envelopesare reviewed and new results for the modelling of the inner wind of the archetype carbonstar IRC+10216 are presented. Benzene, C6H6, forms close to thestar, as well as water, H2O, as a result of non-equilibrium chemistry inducedby the periodic passage of shocks. The growth process of aromatic rings may thus resemblethat 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 thehydrogen-rich R CrB star, V854 Cen, and their implication for the carriers of the RedEmission and the Diffuse Interstellar Bands.