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Register a new userPolycyclic Aromatic Hydrocarbons in Seyfert and star-forming galaxies
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Added by
Ismael Garcia-Bernete Dr.
Publication date
2020
fields
Physics
and research's language is
English
Authors
I. Garcia-Bernete
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Polycyclic Aromatic Hydrocarbons (PAHs) are carbon-based molecules resulting from the union of aromatic rings and related species, which are likely responsible for strong infrared emission features (3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 microns). In this work, using a sample of Seyfert galaxies (DL<100 Mpc), we compare the circumnuclear (inner kpc) PAH emission of AGN and star-forming (SF) control samples, and we investigate the difference between the central and extended PAH properties. We employ newly developed PAH diagnostic model grids, derived from theoretical spectra, to compare the predicted and observed PAH ratios. We use Spitzer/InfraRed Spectrograph spectral data for a large sample of Seyfert galaxies and SF galaxies. In general we find that SF galaxies and powerful Seyfert galaxies are located in different regions of the PAH diagnostic diagram, which indicates that the size and charge of the PAH molecules but also the nature and hardness of the radiation field that excite them are different. Our work indicates that powerful AGN seem to favour larger PAH molecules (Nc>400) as well as neutral species. By subtracting the central from the total spectra we are able to compare the PAH emission in the central/extended region of a small sample of AGN. In contrast with the findings for central regions of AGN-dominated systems, we find that the extended emission of both Seyfert types has similar PAH molecular size distribution and ionized fraction of molecules than in central regions of SF galaxies (100< Nc< 300).
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The abundance of polycyclic aromatic hydrocarbons (PAHs) in low- and high-metallicity galaxies has been widely discussed since the time when detailed infrared data for extragalactic objects were first obtained. On the scales of entire galaxies, a smaller PAH abundance in lower-metallicity galaxies is often observed. We study this relationship for star-forming regions in nearby galaxies, for a sample containing more than 200 HII complexes, using spatially-resolved observations from the Herschel Space Observatory and Spitzer Space Telescope. We use a model for the dust emission to estimate the physical parameters (PAH abundance, metallicity, ultraviolet radiation field, etc.) of these complexes. The same correlation of PAH abundance with metallicity, as seen for entire galaxies, is apparently preserved at smaller scales, at least when the Kobulnicky & Kewley metallicity calibration is used. We discuss possible reasons for this correlation, noting that traces of less-effective PAH formation in low-metallicity AGB stars should be smeared out by radial mixing in galactic disks. Effective destruction by the harder and more intensive ultraviolet field in low-metallicity environments is qualitatively consistent with our data, as the ultraviolet field intensity, derived from the infrared photometry, is indeed smaller in HII complexes with lower metallicity.
The amount of deuterium locked up in polycyclic aromatic hydrocarbons (PAHs) has to date been an uncertain value. We present a near-infrared (NIR) spectroscopic survey of HII regions in the Milky Way, Large Magellanic Cloud (LMC), and Small Magellanic Cloud (SMC) obtained with AKARI, which aims to search for features indicative of deuterated PAHs (PAD or Dn-PAH) to better constrain the D/H ratio of PAHs. Fifty-three HII regions were observed in the NIR (2.5-5 {mu}m), using the Infrared Camera (IRC) on board the AKARI satellite. Through comparison of the observed spectra with a theoretical model of deuterated PAH vibrational modes, the aromatic and (a)symmetric aliphatic C-D stretch modes were identified. We see emission features between 4.4-4.8 {mu}m, which could be unambiguously attributed to deuterated PAHs in only six of the observed sources, all of which are located in the Milky Way. In all cases, the aromatic C-D stretching feature is weaker than the aliphatic C-D stretching feature, and, in the case of M17b, this feature is not observed at all. Based on the weak or absent PAD features in most of the observed spectra, it is suggested that the mechanism for PAH deuteration in the ISM is uncommon.
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 present a new method to accurately describe the ionization fraction and the size distribution of polycyclic aromatic hydrocarbons (PAHs) within astrophysical sources. To this purpose, we have computed the mid-infrared emission spectra of 308 PAH molecules of varying sizes, symmetries, and compactness, generated in a range of radiation fields. We show that the intensity ratio of the solo CH out-of-plane bending mode in PAH cations and anions (referred to as the 11.0 $mu$m band, falling in the 11.0-11.3 $mu$m region for cations and anions) to their 3.3 $mu$m emission, scales with PAH size, similarly to the scaling of the 11.2/3.3 ratio with the number of carbon atoms (N$_{mathrm{C}}$) for neutral molecules. Among the different PAH emission bands, it is the 3.3 $mu$m band intensity which has the strongest correlation with N$_{mathrm{C}}$, and drives the reported PAH intensity ratio correlations with N$_{mathrm{C}}$ for both neutral and ionized PAHs. The 6.2/7.7 intensity ratio, previously adopted to track PAH size, shows no evident scaling with N$_{mathrm{C}}$ in our large sample. We define a new diagnostic grid space to probe PAH charge and size, using the (11.2+11.0)/7.7 and (11.2+11.0)/3.3 PAH intensity ratios respectively. We demonstrate the application of the (11.2+11.0)/7.7 - (11.2+11.0)/3.3 diagnostic grid for galaxies M82 and NGC 253, for the planetary nebula NGC 7027, and the reflection nebulae NGC 2023 and NGC 7023. Finally, we provide quantitative relations for PAH size determination depending on the ionization fraction of the PAHs and the radiation field they are exposed to.
Aromaticity is a well-known phenomenon in both physics and chemistry, and is responsible for many unique chemical and physical properties of aromatic molecules. The primary feature contributing to the stability of polycyclic aromatic hydrocarbons is the delocalised $pi$-electron clouds in the $2p_z$ orbitals of each of the $N$ carbon atoms. While it is known that electrons delocalize among the hybridized $sp^2$ orbitals, this paper proposes quantum walk as the mechanism by which the delocalization occurs, and also obtains how the functional chemical structures of these molecules arise naturally out of such a construction. We present results of computations performed for some benzoid polycyclic aromatic hydrocarbons in this regard, and show that the quantum walk-based approach does correctly predict the reactive sites and stability order of the molecules considered.
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