Polycyclic aromatic hydrocarbons and the unidentified infrared emission bands - Auto exhaust along the Milky Way

Allamandola, Louis J.; Tielens, A. G. G. M.; Barker, John R.

doi:10.1086/184435

Cited by 1,137 publications

(833 citation statements)

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“…Unlike the far-infrared which is dominated by smooth continuum generated through cold dust modified blackbody emission and the occasionally narrow gas emission line, the mid-infrared portion has emission and absorption features generated by heavy molecules and smaller dust grains. Polycyclic Aromatic Hydrocarbons (PAHs) are a few Å-diameter heavy molecules containing hundreds of carbon atoms which exist in cold molecular clouds (Leger & Puget, 1984;Allamandola et al, 1985) and when irradiated by young stars, emit spectral line features at discrete wavelengths from ≈3-19µm (Weingartner & Draine, 2001). It follows that PAH emission strength scales with star formation rate (which relies on the assumption that stellar emission heats the photo-dissociation regions where the PAHs reside Farrah et al, 2007).…”

Section: Mid-infrared Diagnosticsmentioning

confidence: 99%

Dusty star-forming galaxies at high redshift

2014

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Far-infrared and submillimeter wavelength surveys have now established the important role of dusty, star-forming galaxies (DSFGs) in the assembly of stellar mass and the evolution of massive galaxies in the Universe. The brightest of these galaxies have infrared luminosities in excess of 10 13 L with implied star-formation rates of thousands of solar masses per year. They represent the most intense starbursts in the Universe, yet many are completely optically obscured. Their easy detection at submm wavelengths is due to dust heated by ultraviolet radiation of newly forming stars. When summed up, all of the dusty, star-forming galaxies in the Universe produce an infrared radiation field that has an equal energy density as the direct starlight emission from all galaxies visible at ultraviolet and optical wavelengths. The bulk of this infrared extragalactic background light emanates from galaxies as diverse as gas-rich disks to mergers of intense starbursting galaxies. Major advances in far-infrared instrumentation in recent years, both spacebased and ground-based, has led to the detection of nearly a million DSFGs, yet our understanding of the underlying astrophysics that govern the start and end of the dusty starburst phase is still in nascent stage. This review is aimed at summarizing the current status of DSFG studies, focusing especially on the detailed characterization of the bestunderstood subset (submillimeter galaxies, who were summarized in the last review of this field over a decade ago, Blain et al. 2002), but also the selection and characterization of more recently discovered DSFG populations. We review DSFG population statistics, their physical properties including dust, gas and stellar contents, their environments, and current theoretical models related to the formation and evolution of these galaxies.

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Section: Mid-infrared Diagnosticsmentioning

confidence: 99%

Dusty star-forming galaxies at high redshift

2014

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“…The idea now gaining acceptance, that stochastically heated, gas-phase, polycyclic aromatic hydrocarbons (PAHs) are the band carriers, was first put forth over a decade ago [12,13]. This attribution is based on several pieces of 'circumstantial' evidence that point to these species.…”

Section: Introductionmentioning

confidence: 99%

“…Unfortunately, although rich, this spectral database was not adequate to squarely address the astronomical questions since such laboratory conditions strongly perturb the spectrum and are far from those in the interstellar emission zones. Moreover, if PAHs are indeed present in the highly energetic emission zones, they are likely to be ionized [13] and, until recently, there was no spectroscopic data available on the infrared properties of PAH ions. Thus, the spectral database initially available was not sufficient for a critical test of the PAH hypothesis nor, if the hypothesis held up to close scrutiny, was it up to the task of exploiting PAHs as probes of the emission zones.…”

Section: Introductionmentioning

confidence: 99%

Closed-shell polycyclic aromatic hydrocarbon cations: a new category of interstellar polycyclic aromatic hydrocarbons

Hudgins

Bauschlicher

Allamandola

2001

Spectrochimica Acta Part A: Molecular and Biomolecular Spectros

111

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Density functional theory has been employed to calculate the harmonic frequencies and intensities of a range of polycyclic aromatic hydrocarbon (PAH) cations that explore both size and electronic structure effects on the infrared spectroscopic properties of these species. The sample extends the size range of PAH species considered to more than 50 carbon atoms and includes several representatives from each of two heretofore unexplored categories of PAH cations: (1) fully benzenoid PAH cations whose carbon skeleton is composed of an odd number of carbon atoms (C odd PAHs); and (2) protonated PAH cations (HPAH + ). Unlike the radical electronic structures of the PAH cations that have been the subject of previous theoretical and experimental work, the species in these two classes have a 'closed'-shell electronic configuration. The calculated spectra of circumcoronene, C 54 H 18 , in both neutral and (radical) cationic form are also reported and compared with those of the other species. Overall, the C odd PAHs spectra are dominated by strong CC stretching modes near 1600 cm − 1 and display spectra that are remarkably insensitive to molecular size. The HPAH + species evince a more complex spectrum consistent with the added contributions of aliphatic modes and their generally lower symmetry. Finally, for both classes of closed-shell cations, the intensity of the aromatic CH stretching modes is found to increase with molecular size far out of proportion with the number of CH groups, approaching a value more typical of neutral PAHs for the largest species studied.

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“…Calculations show that the carriers of these features are as abundant as the most abundant simple polyatomic molecule, NH 3 (10 -6 -10 -7 with respect to hydrogen; e.g. [19,20]). Remarkably, recent ISO observations have even indicated that these features are sufficiently prevalent and intense in distant galaxies that they might be useful as red-shift indicators (e.g.…”

Section: Interstellar Pahs: the Observational Foundationmentioning

confidence: 99%

“…[19,20] The rationale underlying this suggestion is straightforward. First, the emission bands are non-thermal in nature -that is, they are observed even in regions where the dust temperature is too low for the material to be emitting thermally.…”

Section: Interstellar Pahs: the Observational Foundationmentioning

confidence: 99%

From Interstellar Polycyclic Aromatic Hydrocarbons and Ice to Astrobiology

Allamandola

Hudgins

2003

Solid State Astrochemistry

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Tremendous strides have been made in our understanding of interstellar material over the past twenty years thanks to significant, parallel developments in observational astronomy and laboratory astrophysics. Twenty years ago the composition of interstellar dust was largely guessed at, the concept of ices in dense molecular clouds ignored, and the notion of large, abundant, gas phase, carbon rich molecules widespread throughout the interstellar medium (ISM) considered impossible.Today the composition of dust in the diffuse ISM is reasonably well constrained to micron-sized cold refractory materials comprised of amorphous and crystalline silicates mixed with an amorphous carbonaceous material containing aromatic structural units and short, branched aliphatic chains. In dense molecular clouds, the birthplace of stars and planets, these cold dust particles are coated with mixed molecular ices whose major components are is very well constrained. Lastly, the signature of carbon-rich polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by earlier interstellar chemistry standards, is widespread throughout the Universe. This paper presents a detailed summary of these disparate interstellar components and ends by considering both of them as important feedstock to the chemical inventory of the primordial Earth. Particular attention is paid to their possible role in the chemistry that led to the origin of life. An extensive reference list is given to allow the student entry into the full depth of the literature.The first part of this paper focuses on interstellar PAHs. The laboratory and theoretical underpinning which supports the PAH model is reviewed in some detail. This is followed by a few specific examples which demonstrate how these data can be used to analyze the interstellar spectra and probe local conditions in different emission zones. These examples include tracing the evolution of carbon as it passes from its birthsite in circumstellar shells through the ISM, determining specifics about the cosmic PAH population in many different environments including PAH size and structure, and probing local conditions in the different emission zones.The second part of this paper summarizes the laboratory and observational background leading to our current understanding of interstellar/precometary ices. Although the most abundant interstellar ice components are the very simple molecules such as H 2 O, CH 3 OH, CO, CO 2 , and NH 3 , more complex species including accreted PAHs and those formed by UV and cosmic ray processing within the ice must also be present. Here we give a detailed summary of the photochemical evolution on those ices 2 found in the densest regions of molecular clouds, the regions where stars and planetary systems are formed. Ultraviolet photolysis of these ices produces a host of new compounds, some of which show intriguing prebiotic behavior.The last part of this paper draws all this information together and considers the possible roles these compounds might have played in early Earth...

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Polycyclic aromatic hydrocarbons and the unidentified infrared emission bands - Auto exhaust along the Milky Way

Cited by 1,137 publications

References 0 publications

Dusty star-forming galaxies at high redshift

Dusty star-forming galaxies at high redshift

Closed-shell polycyclic aromatic hydrocarbon cations: a new category of interstellar polycyclic aromatic hydrocarbons

From Interstellar Polycyclic Aromatic Hydrocarbons and Ice to Astrobiology

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