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Register a new userIRAC Excess in Distant Star-Forming Galaxies: Tentative Evidence for the 3.3$mu$m Polycyclic Aromatic Hydrocarbon Feature ?
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We present evidence for the existence of an IRAC excess in the spectral energy distribution (SED) of 5 galaxies at 0.6<z<0.9 and 1 galaxy at z=1.7. These 6 galaxies, located in the Great Observatories Origins Deep Survey field (GOODS-N), are star forming since they present strong 6.2, 7.7, and 11.3 um polycyclic aromatic hydrocarbon (PAH) lines in their Spitzer IRS mid-infrared spectra. We use a library of templates computed with PEGASE.2 to fit their multiwavelength photometry and derive their stellar continuum. Subtraction of the stellar continuum enables us to detect in 5 galaxies a significant excess in the IRAC band pass where the 3.3 um PAH is expected. We then assess if the physical origin of the IRAC excess is due to an obscured active galactic nucleus (AGN) or warm dust emission. For one galaxy evidence of an obscured AGN is found, while the remaining four do not exhibit any significant AGN activity. Possible contamination by warm dust continuum of unknown origin as found in the Galactic diffuse emission is discussed. The properties of such a continuum would have to be different from the local Universe to explain the measured IRAC excess, but we cannot definitively rule out this possibility until its origin is understood. Assuming that the IRAC excess is dominated by the 3.3 um PAH feature, we find good agreement with the observed 11.3 um PAH line flux arising from the same C-H bending and stretching modes, consistent with model expectations. Finally, the IRAC excess appears to be correlated with the star-formation rate in the galaxies. Hence it could provide a powerful diagnostic for measuring dusty star formation in z>3 galaxies once the mid-infrared spectroscopic capabilities of the James Webb Space Telescope become available.
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Astronomical mid-IR spectra show two minor PAH features at 5.25 and 5.7 $mu$m (1905 and 1754 cm$^{rm - 1}$) that hitherto have been little studied, but contain information about the astronomical PAH population that complements that of the major emission bands. Here we report a study involving both laboratory and theoretical analysis of the fundamentals of PAH spectroscopy that produce features in this region and use these to analyze the astronomical spectra. The ISO SWS spectra of fifteen objects showing these PAH features were considered for this study, of which four have sufficient S/N between 5 and 6 $mu$m to allow for an in-depth analysis. All four astronomical spectra show similar peak positions and profiles. The 5.25 $mu$m feature is peaked and asymmetric, while the 5.7 $mu$m feature is broader and flatter. Detailed analysis of the laboratory spectra and quantum chemical calculations show that the astronomical 5.25 and 5.7 $mu$m bands are a blend of combination, difference and overtone bands primarily involving CH stretching and CH in-plane and CH out-of-plane bending fundamental vibrations. The experimental and computational spectra show that, of all the hydrogen adjacency classes possible on PAHs, solo and duo hydrogens consistently produce prominent bands at the observed positions whereas quartet hydrogens do not. In all, this a study supports the picture that astronomical PAHs are large with compact, regular structures. From the coupling with primarily strong CH out-of-plane bending modes one might surmise that the 5.25 and 5.7 $mu$m bands track the neutral PAH population. However, theory suggests the role of charge in these astronomical bands might also be important.
We have examined polycyclic aromatic hydrocarbon (PAH) excitation in a sample of 25 nearby face-on spiral galaxies using the ratio of mid-infrared PAH emission to dust mass. Within 11 of the galaxies, we found that the PAH excitation was straightforwardly linked to ultraviolet or mid-infrared star formation tracers, which, along with other results studying the relation of PAH emission to star formation, indicates that the PAHs are most strongly excited in dusty shells around the star forming regions. Within another 5 galaxies, the PAH emission is enhanced around star forming regions only at specific galactocentric radii. In 6 more galaxies, PAH excitation is more strongly correlated with the evolved stellar populations as traced by 3.6 micron emission. The results for the remaining 3 galaxies were ambiguous. The radial gradients of the PAH/dust ratios were generally not linked to log(O/H) gradients except when the log(O/H) gradients were relatively steep. Galaxies in which PAHs were excited by evolved stars had relatively high far-ultraviolet to mid-infrared ratios, indicating that variations in the link between PAH excitation and different stellar populations is linked to changes in dust attenuation within galaxies. Alternately, differences in morphology could make it more likely that PAHs are excited by evolved stars, as 5 of the 6 galaxies where this occurs are late-type flocculent spiral galaxies. These heterogeneous results demonstrate the complexity of describing PAH excitation and have broad implications for using PAH emission as a star formation tracer as well as for modelling dust emission and radiative transfer.
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).
We report new properties of the 11 and 12.7 {mu}m emission complexes of polycyclic aromatic hydrocarbons (PAHs) by applying a Gaussian-based decomposition technique. Using high-resolution textit{Spitzer} Space Telescope data, we study in detail the spectral and spatial characteristics of the 11 and 12.7 {mu}m emission bands in maps of reflection nebulae NGC 7023 and NGC 2023 (North and South) and the star-forming region M17. Profile variations are observed in both the 11 and 12.7 {mu}m emission bands. We identify a neutral contribution to the traditional 11.0 {mu}m PAH band and a cationic contribution to the traditional 11.2 {mu}m band, the latter of which affects the PAH class of the 11.2 {mu}m emission in our sample. The peak variations of the 12.7 {mu}m complex are explained by the competition between two underlying blended components. The spatial distributions of these components link them to cations and neutrals. We conclude that the 12.7 {mu}m emission originates in both neutral and cationic PAHs, lending support to the use of the 12.7/11.2 intensity ratio as a charge proxy.
Using the AKARI, Wide-field Infrared Survey Explorer (WISE), Infrared Astronomical Satellite (IRAS), Sloan Digital Sky Survey (SDSS) and Hubble Space Telescope (HST) data, we investigated the relation of polycyclic aromatic hydrocarbon (PAH) mass ($M_{rm PAH}$), very small grain mass ($M_{rm VSG}$), big grain mass ($M_{rm BG}$) and stellar mass ($M_{rm star}$) with galaxy merger for 55 star-forming galaxies at redshift $z<0.2$. Using the SDSS image at $z<0.1$ and the HST image at $z>0.1$, we divided the galaxies into merger galaxies and non-merger galaxies with the morphological parameter asymmetry $A$, and quantified merging stages of galaxies based on the morphological indicators, the second-order momentum of the brightest 20$%$ region $M_{20}$ and the Gini coefficient. We find that $M_{rm PAH}/M_{rm BG}$ of merger galaxies tend to be lower than that of non-merger galaxies and there are no systematic differences of $M_{rm VSG}/M_{rm BG}$ and $M_{rm BG}/M_{rm star}$ between merger galaxies and non-merger galaxies. We find that galaxies with very low $M_{rm PAH}/M_{rm BG}$ seem to be merger galaxies at late stages. These results suggest that PAHs are partly destroyed at late stages of merging processes. Furthermore, we investigated $M_{rm PAH}/M_{rm BG}$ variations in radiation field intensity strength $G_0$ and the emission line ratio of $[{rm O,{scriptsize I}}]lambda 6300/{rm H}alpha$ which is a shock tracer for merger galaxies and find that $M_{rm PAH}/M_{rm BG}$ decreases with increasing both $G_0$ and $[{rm O,{scriptsize I}}]/{rm H}alpha$. PAH destruction is likely to be caused by two processes; strong radiation fields and large-scale shocks during merging processes of galaxies.
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