Polycyclic Aromatic Hydrocarbon Emission Features Research Articles

The need for spatially resolved observations of polycyclic aromatic hydrocarbons in protoplanetary discs

Context. The signatures of polycyclic aromatic hydrocarbons (PAHs) have been observed in protoplanetary discs, and their emission features obtained from spectral energy distributions (SED) have been used in the literature to characterise their size and determine their abundance. Aims. Two simple disc models (uniform PAH distribution against a PAH gap in the inner disc) are compared to investigate the difference of their SED and obtainable information. Methods. We used the radiative transfer code RADMC-3D to model the SED of two protoplanetary discs orbiting a typical Herbig star, one of which features a depletion of PAHs in the inner disc. We further created artificial images of the discs at face-on view to extract radial profiles of the PAH emission in the infrared. Results. We find that the extracted PAH features from an SED provide limited information about the PAHs in protoplanetary disc environments, except for the ionisation state. The distribution of PAHs in a protoplanetary disc influences the total observed PAH luminosity in a non-linear fashion and alters the relative strength between the 3.3 µm and 11.3 µm features. Furthermore, we produced radial profiles at the 3 µm, 6 µm and, 11 µm PAH emission features and find that they follow a double power-law profile where the slope reflects the radiative environment (single photon regime vs. multi-photon regime) in which the PAHs lie. Conclusions. Using spatially resolved techniques such as IFU or imaging in the era of the James Webb Space Telescope, we find that multi-wavelength radial emission profiles will not only provide information on the spatial distribution of the PAHs, but may also provide information on their size and underlying UV environment, which is crucial for photo-evaporative disc wind models.

A Diverse Population of z ∼ 2 ULIRGs Revealed by JWST Imaging

Four ultraluminous infrared galaxies (ULIRGs) observed with JWST/NIRcam in the Cosmos Evolution Early Release Science program offer an unbiased preview of the z ∼ 2 ULIRG population. The objects were originally selected at 24 μm and have strong polycyclic aromatic hydrocarbon emission features observed with Spitzer/Infrared Spectrometer. The four objects have similar stellar masses of ∼1011 M ⊙ but otherwise are quite diverse. One is an isolated disk galaxy, but it has an active nucleus as shown by X-ray observations and by a bright point-source nucleus. Two others are merging pairs with mass ratios of 6–7:1. One has active nuclei in both components, while the other has only one active nucleus: the one in the less-massive neighbor, not the ULIRG. The fourth object is clumpy and irregular and is probably a merger, but there is no sign of an active nucleus. The intrinsic spectral energy distributions for the four active galactic nuclei in these systems are typical of type-2 QSOs. This study is consistent with the idea that even if internal processes can produce large luminosities at z ∼ 2, galaxy merging may still be necessary for the most luminous objects. The diversity of these four initial examples suggests that large samples will be needed to understand the z ∼ 2 ULIRG population.

PHANGS–JWST First Results: Measuring Polycyclic Aromatic Hydrocarbon Properties across the Multiphase Interstellar Medium

Ratios of polycyclic aromatic hydrocarbon (PAH) vibrational bands are a promising tool for measuring the properties of the PAH population and their effect on star formation. The photometric bands of the MIRI and NIRCam instruments on JWST provide the opportunity to measure PAH emission features across entire galaxy disks at unprecedented resolution and sensitivity. Here we present the first results of this analysis in a sample of three nearby galaxies: NGC 628, NGC 1365, and NGC 7496. Based on the variations observed in the 3.3, 7.7, and 11.3 μm features, we infer changes to the average PAH size and ionization state across the different galaxy environments. High values of F335MPAH/F1130W and low values of F1130W/F770W are measured in H ii regions in all three galaxies. This suggests that these regions are populated by hotter PAHs, and/or that the PAH ionization fraction is larger. We see additional evidence of heating and/or changes in PAH size in regions with higher molecular gas content as well as increased ionization in regions with higher Hα intensity.

Evolution of grain size distribution with enhanced abundance of small carbonaceous grains in galactic environments

ABSTRACT We propose an updated dust evolution model that focuses on the grain size distribution in a galaxy. We treat the galaxy as a one-zone object and include five main processes (stellar dust production, dust destruction in supernova shocks, grain growth by accretion and coagulation, and grain disruption by shattering). In this paper, we improve the predictions related to small carbonaceous grains, which are responsible for the 2175-Å bump in the extinction curve and the polycyclic aromatic hydrocarbon emission features in the dust emission spectral energy distribution (SED), both of which were underpredicted in our previous model. In the new model, we hypothesize that small carbonaceous grains are not involved in interstellar processing. This avoids small carbonaceous grains being lost by coagulation. We find that this hypothetical model shows a much better match to the Milky Way (MW) extinction curve and dust emission SED than the previous one. The following two additional modifications further make the fit to the MW dust emission SED better (i) The chemical enrichment model is adjusted to give a nearly solar metallicity in the present epoch, and the fraction of metals available for dust growth is limited to half. (ii) Aromatization for small carbonaceous grains is efficient, so that the aromatic fraction is unity at grain radii ≲ 20 Å. As a consequence of our modelling, we succeed in obtaining a dust evolution model that explains the MW extinction curve and dust emission SED at the same time.

Relations between Mid-infrared Dust Emission and UV Extinction

Abstract We analyze low-resolution Spitzer infrared (IR) 5−14 μm spectra of the diffuse emission toward a carefully selected sample of stars. The sample is composed of sight lines toward stars that have well-determined ultraviolet (UV) extinction curves and that are shown to lie beyond effectively all of the extinguishing and emitting dust along their lines of sight. Our sample includes sight lines whose UV curve extinction curves exhibit a wide range of curve morphology and that sample a variety of interstellar environments. As a result, this unique sample enabled us to study the connection between the extinction and emission properties of the same grains, and to examine their response to different physical environments. We quantify the emission features in terms of the polycyclic aromatic hydrocarbon (PAH) model given by Draine & Li and a set on additional features not known to be related to PAH emission. We compare the intensities of the different features in the Spitzer mid-infrared spectra with the Fitzpatrick & Massa parameters that describe the shapes of UV to near-infrared extinction curves. Our primary result is that there is a strong correlation between the area of the 2175 Å UV bump in the extinction curves of the program stars and the strengths of the major PAH emission features in the mid-infrared spectra for the same lines of sight.

Excitation of Polycyclic Aromatic Hydrocarbon Emission: Dependence on Size Distribution, Ionization, and Starlight Spectrum and Intensity

Abstract Using physical models, we study the sensitivity of polycyclic aromatic hydrocarbon (PAH) emission spectra to the character of the illuminating starlight, to the PAH size distribution, and to the PAH charge distribution. The starlight models considered range from the emission from a 3 Myr old starburst, rich in far-ultraviolet (FUV) radiation, to the FUV-poor spectrum of the very old population of the M31 bulge. A wide range of starlight intensities is considered. The effects of reddening in dusty clouds are investigated for different starlight spectra. For a fixed PAH abundance parameter q PAH (the fraction of the total dust mass in PAHs with <103 C atoms), the fraction of the infrared power appearing in the PAH emission features can vary by a factor of two as the starlight spectrum varies from FUV-poor (M31 bulge) to FUV-rich (young starburst). We show how q PAH can be measured from the strength of the 7.7 μm emission. The fractional power in the 17 μm feature can be suppressed by high starlight intensities.

Tug of War, or Cohabitation? : Star Formation and AGN Activities within Type 1 AGN Host Galaxies

AbstractThe interplay between star formation (SF) activity and active galactic nuclei (AGN) governs the co-evolution of supermassive black holes (SMBHs) and their host galaxies. AGN feedback has been hailed as the de facto process to suppress, or even shut down SF within the framework of hierarchical galaxy merger based on the current ΛCDM paradigm. However, it is unclear what physical processes regulate the growth of SMBHs and how SMBHs and their evolution are interconnected with their host galaxies when SMBHs and host galaxies are of hugely different physical scales. In fact, there has been no observational evidence to show that AGN feedback works, but rather some evidence to speculate that the more powerful AGNs reside in the more actively star-forming host galaxies. While it is difficult to measure the amount of SF from AGN host galaxies, polycyclic aromatic hydrocarbon (PAH) emission features emerged as good proxies for this purpose. Although having several caveats as SFR indicators, such as metallicity dependency, and non-SF contribution from evolved stellar populations, or AGNs, PAH emissions have been utilized to investigate SF activity of AGN host galaxies with varying results. Utilizing the slitless spectroscopic apability of the AKARI Infrared Camera, we obtained the spectra in the wavelength range of 2∼5 μm from extended regions of 79 type 1 AGN host galaxies to detect and measure the 3.3 μm (PAH) emission feature as star formation rate proxy. Based on 18 sample galaxies, we found that the luminosity of the 3.3 μm PAH emission feature is strongly correlated with AGN luminosity, except for ultra-luminous infrared galaxies (ULIRGs). Therefore, we suggest that host galaxies with stronger AGN activities have stronger star formation activities. However, it is still unclear why ULIRGs deviate from the correlation, not to mention why the detection rate of the 3.3 μm emission feature is so low. High spatial resolution imaging not only for the circumnuclear region of AGN host galaxies, but also for entire galaxies should help the cause. We present the prospective studies to diagnose SF regulation for AGN host galaxies with various space telescope facilities, such as JWST, and SPHEREx.

A Survey of 3–5.4 μm Emission from Planetary Nebulae Using SOFIA/FLITECAM

Abstract Here we present the results of an airborne 3–5.4 μm spectroscopic study of three young, carbon-rich planetary nebulae (PNs) IC 5117, PNG 093.9-00.1, and BD +30 3639. These observations were made using the grism spectroscopy mode of the First Light Infrared TEst CAMera (FLITECAM) instrument during airborne science operations on board NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA). The goal of this study is to characterize the 3.3 and 5.25 μm polycyclic aromatic hydrocarbon (PAH) dust emission in PNs and study the evolution of PAH features within evolved stars before their incorporation into new stellar systems in star-forming regions. Targets were selected from Infrared Astronomical Satellite, Kuiper Airborne Observatory and Infrared Space Observatory source lists, and were previously observed with FLITECAM on the 3 m Shane telescope at Lick Observatory to allow direct comparison between the ground and airborne observations. We measure PAH emission equivalent width and central wavelength, classify the shape of the PAH emission, and determine the PAH/aliphatic ratio for each target. The 3.3 μm PAH emission feature is observed in all three objects. PNG 093.9-00.1 exhibits NGC 7027-like aliphatic emission in the 3.4–3.6 μm region while IC 5117 and BD +30 3639 exhibit less aliphatic structure. All three PNs additionally exhibit PAH emission at 5.25 μm.

The interplay between active galactic nuclei and star formation activities of type 1 active galactic nuclei probed by polycyclic aromatic hydrocarbon 3.3 μm emission feature with AKARI

Abstract We investigate the connection between active galactic nucleus (AGN) and star formation activities of AGN host galaxies by studying the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission feature of 79 type 1 AGNs using the AKARI space telescope. Utilizing the slitless spectroscopic capability of the AKARI Infrared Camera, we obtained the spectra in the wavelength range of 2–5 μm from extended regions of the sample galaxies in order to measure star formation activity from the entire host galaxies. We detected the 3.3 μm PAH emission feature from 18 sample galaxies and measured the luminosities of the feature (LPAH3.3). We found that LPAH3.3 is significantly correlated with AGN luminosities (LAGN), such as 5100 Å monochromatic luminosity, and X-ray luminosity regardless of host galaxy morphology and radio-loudness. The correlation between LPAH3.3 and LAGN follows $L_{\rm {PAH3.3}} \propto L_{\rm {AGN}}^{0.9}$. Therefore we suggest that host galaxies with stronger AGN activities have stronger star formation activities. We also found that the ratios between LPAH3.3 and the bolometric infrared luminosity (LIR) of our sample galaxies are lower than for non-AGN galaxies due to increased LIR. We suggest that this can be attributed to the contribution of AGN to LIR.

Effect of Alignment on Polarized Infrared Emission from Polycyclic Aromatic Hydrocarbons

Abstract Polarized emission from polycyclic aromatic hydrocarbons (PAHs) potentially provides a new way to test the basic physics of the alignment of ultrasmall grains. In this paper, we present a new model of polarized PAH emission that takes into account the effect of PAH alignment with the magnetic field. We first generate a large sample of the grain angular momentum by simulating the alignment of PAHs due to resonance paramagnetic relaxation that accounts for various interaction processes. We then calculate the polarization level of the PAH emission features for the different phases of the interstellar medium, including the cold neutral medium (CNM), reflection nebulae (RNe), and photodissociation regions. We find that a moderate degree of PAH alignment can significantly enhance the polarization degree of the PAH emission compared to the previous results obtained with randomly oriented angular momentum. In particular, we find that the smallest negatively charged PAHs in RNe can be excited to slightly suprathermal rotation due to enhanced ion collisional excitation, resulting in an increase of the polarization with the ionization fraction. Our results suggest that an RN is the most favorable environment in which to observe polarized PAH emission and to test the alignment physics of nanoparticles. Finally, we present an explicit relationship between the polarization level of PAH emission and the degree of external alignment for the CNM and RNe. The obtained relationship will be particularly useful for testing the alignment physics of PAHs in future observations.

SPATIAL VARIATIONS OF PAH PROPERTIES IN M17SW REVEALED BY SPITZER/IRS SPECTRAL MAPPING

ABSTRACT We present Spitzer/IRS mid-infrared spectral maps of the Galactic star-forming region M17 as well as IRSF/SIRIUS Brγ and Nobeyama 45 m/FOREST 13CO (J = 1–0) maps. The spectra show prominent features due to polycyclic aromatic hydrocarbons (PAHs) at wavelengths of 6.2, 7.7, 8.6, 11.3, 12.0, 12.7, 13.5, and 14.2 μm. We find that the PAH emission features are bright in the region between the H ii region traced by Brγ and the molecular cloud traced by 13CO, supporting that the PAH emission originates mostly from photo-dissociation regions. Based on the spatially resolved Spitzer/IRS maps, we examine spatial variations of the PAH properties in detail. As a result, we find that the interband ratio of PAH 7.7 μm/PAH 11.3 μm varies locally near M17SW, but rather independently of the distance from the OB stars in M17, suggesting that the degree of PAH ionization is mainly controlled by local conditions rather than the global UV environments determined by the OB stars in M17. We also find that the interband ratios of the PAH 12.0 μm, 12.7 μm, 13.5 μm, and 14.2 μm features to the PAH 11.3 μm feature are high near the M17 center, which suggests structural changes of PAHs through processing due to intense UV radiation, producing abundant edgy irregular PAHs near the M17 center.

Sources and mass inventory of sedimentary polycyclic aromatic hydrocarbons in the Gulf of Thailand: Implications for pathways and energy structure in SE Asia

Surface sediments obtained from a matrix of 92 sample sites in the Gulf of Thailand (GOT) were analyzed for a comprehensive study of the distribution, sources, and mass inventory of polycyclic aromatic hydrocarbons (PAHs) to assess their input pathways and impacts of the regional land-based energy structure on the deposition of PAHs on the adjacent continental margins. The concentration of 16 PAHs in the GOT ranged from 2.6 to 78.1ng/g (dry weight), and the mean concentration was 19.4±15.1ng/g. The spatial distribution pattern of 16 PAH was generally consistent with that of sediment grain size, suggesting the influence of regional hydrodynamic conditions. Correlation and principal component analysis of the PAHs indicated that direct land-based inputs were dominantly responsible for the occurrence of PAHs in the upper GOT and the low molecular weight (LMW) PAHs in the coastal region could be from petrogenic sources. A positive matrix factorization (PMF) model apportioned five contributors: petroleum residues (~44%), biomass burning (~13%), vehicular emissions (~11%), coal combustion (~6%), and air–water exchange (~25%). Gas absorption may be a significant external input pathway for the volatile PAHs in the open GOT, which further implies that atmospheric loading could be important for the sink of PAHs in the open sea of the Southeast Asia (SE Asia). The different PAH source patterns obtained and a significant disparity of PAH mass inventory in the sediments along the East and Southeast Asia continental margins can be ascribed mainly to different land-based PAH emission features under the varied regional energy structure in addition to the depositional environment and climatic conditions.

A tale of three galaxies: A “CLUMPY” view of the spectroscopically anomalous galaxies IRAS F10398+1455, IRAS F21013–0739 and SDSS J0808+3948

We investigate the dust properties in three spectroscopically anomalous galaxies (IRAS F10398+1455, IRAS F21013–0739 and SDSS J0808+3948). Their Spitzer/IRS spectra are characterized by a steep ∼5–8μm emission continuum, strong emission bands from polycyclic aromatic hydrocarbon (PAH) molecules, and prominent 10μm silicate emission. The steep ∼5–8μm continuum and strong PAH emission features suggest the presence of starbursts, while the silicate emission is indicative of significant heating from AGNs. The simultaneous detection of these two observational properties has rarely been reported on galactic scale. We employ the PAHFIT software to estimate their starlight contributions, and the CLUMPY model for the components contributed by the AGN tori. We find that the CLUMPY model is generally successful in explaining the overall dust infrared emission, although it appears to emit too flat at the ∼5–8μm continuum to be consistent with that observed in IRAS F10398+1455 and IRAS F21013–0739. The flat ∼5–8μm continuum calculated from the CLUMPY model could arise from the adopted specific silicate opacity of Ossenkopf et al. (1992) which exceeds that of the Draine and Lee (1984) “astronomical silicate” by a factor up to 2 in the ∼5–8μm wavelength range. Future models with a variety of dust species incorporated in the CLUMPY radiation transfer regime are needed for a thorough understanding of the dust properties of these spectroscopically anomalous galaxies.

DUST AND POLYCYCLIC AROMATIC HYDROCARBON IN THE PRE-TRANSITIONAL DISK AROUND HD 169142

ABSTRACT The pre-transitional disk around the Herbig Ae star HD 169142 shows a complex structure of possible ongoing planet formation in dust thermal emission from the near-infrared (IR) to millimeter wavelength range. Also, a distinct set of broad emission features at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 &mgr; m ?> , commonly attributed to polycyclic aromatic hydrocarbons (PAHs), is detected prominently in the HD 169142 disk. We model the spectral energy distribution (SED) as well as the PAH emission features of the HD 169142 disk simultaneously with porous dust and astronomical-PAHs, taking into account the spatially resolved disk structure. Our porous dust model, consisting of three distinct components that are primarily concentrated in the inner ring, middle ring, and outer disk, provides an excellent fit to the entire SED, and the PAH model closely reproduces the observed PAH features. The accretion of ice mantles onto porous dust aggregates occurs between ∼16 and 60 AU ?> , which overlaps with the spatial extent (∼ 50 AU ?> ) of the observed PAH emission features. Finally, we discuss the role of PAHs in the formation of planets possibly taking place in the HD 169142 system.

DUST AND POLYCYCLIC AROMATIC HYDROCARBON IN THE HD 34700 DEBRIS DISK

The debris disk around the Vega-type star HD 34700 is detected in dust thermal emission from the near-infrared (IR) to millimeter (mm) and submm wavelength range. Also detected is a distinct set of emission features at 3.3, 6.2, 7.7, 8.6, 11.3, and , which are commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules. We model the observed dust IR spectral energy distribution (SED) and PAH emission features of the HD 34700 disk in terms of porous dust and astronomical-PAHs. Porous dust together with a mixture of neutral and ionized PAHs closely explains the dust IR SED and PAH emission features observed in the HD 34700 disk. Due to the stellar radiation pressure and Poynting–Robertson drag together with the photodissociation of PAHs, substantial removal of dust and PAHs has occurred in the disk, and continuous replenishment of these materials is required to maintain their current abundances. This implies that these materials are not primitive but secondary products probably originating from mutual collisions among planetesimals, asteroids, and comets.

A TALE OF THREE GALAXIES: DECIPHERING THE INFRARED EMISSION OF THE SPECTROSCOPICALLY ANOMALOUS GALAXIES IRAS F10398+1455, IRAS F21013–0739, AND SDSS J0808+3948

The \textit{Spitzer}/Infrared Spectrograph spectra of three spectroscopically anomalous galaxies (IRAS~F10398+1455, IRAS~F21013-0739 and SDSS~J0808+3948) are modeled in terms of a mixture of warm and cold silicate dust, and warm and cold carbon dust. Their unique infrared (IR) emission spectra are characterized by a steep $\simali$5--8$\mum$ emission continuum, strong emission bands from polycyclic aromatic hydrocarbon (PAH) molecules, and prominent silicate emission. The steep $\simali$5--8$\mum$ emission continuum and strong PAH emission features suggest the dominance of starbursts, while the silicate emission is indicative of significant heating from active galactic nuclei (AGNs). With warm and cold silicate dust of various compositions ("astronomical silicate," amorphous olivine, or amorphous pyroxene) combined with warm and cold carbon dust (amorphous carbon, or graphite), we are able to closely reproduce the observed IR emission of these %spectroscopically anomalous galaxies. We find that the dust temperature is the primary cause in regulating the steep $\sim$5--8$\mum$ continuum and silicate emission, insensitive to the exact silicate or carbon dust mineralogy and grain size $a$ as long as $a\simlt1\mum$. More specifically, the temperature of the $\simali$5--8$\mum$ continuum emitter (which is essentially carbon dust) of these galaxies is $\sim$250--400$\K$, much lower than that of typical quasars which is $\sim$640$\K$. Moreover, it appears that larger dust grains are preferred in quasars. The lower dust temperature and smaller grain sizes inferred for these three galaxies compared with that of quasars could be due to the fact that they may harbor a young/weak AGN which is not maturely developed yet.

POLYCYCLIC AROMATIC HYDROCARBON AND MID-INFRARED CONTINUUM EMISSION IN Az> 4 SUBMILLIMETER GALAXY

We report the detection of 6.2um polycyclic aromatic hydrocarbon (PAH) and rest-frame 4-7um continuum emission in the z=4.055 submillimeter galaxy GN20, using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. This represents the first detection of PAH emission at z>4. The strength of the PAH emission feature is consistent with a very high star formation rate of ~1600Msun/yr. We find that this intense starburst powers at least ~1/3 of the faint underlying 6um continuum emission, with an additional, significant (and perhaps dominant) contribution due to a power-law-like hot dust source, which we interpret to likely be a faint, dust-obscured active galactic nucleus (AGN). The inferred 6um AGN continuum luminosity is consistent with a sensitive upper limit on the hard X-ray emission as measured by the Chandra X-Ray Observatory if the previously undetected AGN is Compton-thick. This is in agreement with the finding at optical/infrared wavelengths that the galaxy and its nucleus are heavily dust-obscured. Despite the strong power-law component enhancing the mid-infrared continuum emission, the intense starburst associated with the photon-dominated regions that give rise to the PAH emission appears to dominate the total energy output in the infrared. GN20 is one of the most luminous starburst galaxies known at any redshift, embedded in a rich protocluster of star-forming galaxies. This investigation provides an improved understanding of the energy sources that power such exceptional systems, which represent the extreme end of massive galaxy formation at early cosmic times.

A Relation of the PAH3.3μm Feature with Star-forming Activity for Galaxies with aWide Range of Infrared Luminosity

Abstract For star-forming galaxies, we investigated a global relation between the polycyclic aromatic hydrocarbon (PAH) emission luminosity at 3.3μm, LPAH3.3, and the infrared (8–1000μm) luminosity, LIR, to understand how the PAH3.3μm feature relates to star-formation activity. With AKARI, we performed near-infrared (2.5–5μm) spectroscopy of 184 galaxies having LIR ∼ 108–1013L⊙. We classified the samples into infrared galaxies (IRGs: LIR < 1011L⊙), luminous infrared galaxies (LIRGs: LIR ∼ 1011–1012L⊙) and ultra-luminous infrared galaxies (ULIRGs: LIR > 1012L⊙). We excluded sources likely to be contaminated by AGN activity, based on the restframe equivalent width of the PAH emission feature (<40 nm) and the power-law index, representing the slope of continuum emission (Γ > 1; Fv ∝ λΓ). Of these samples, 13 IRGs, 67 LIRGs, and 20 ULIRGs show the PAH emission feature at λrest = 3.3μm in their spectra. We find that the LPAH3.3/LIR ratio considerably decreases toward the luminous end. Utilizing the mass and temperature of dust grains as well as the Brα emission for the galaxies, we discuss the cause of the relative decrease in the PAH emission with LIR.

The Infrared Spectra of Polycyclic Aromatic Hydrocarbons with Excess Peripheral H Atoms (Hn-PAHs) and their Relation to the 3.4 and 6.9 µm PAH Emission Features.

Polycyclic aromatic hydrocarbons (PAHs) are likely responsible for the family of infrared emission features seen in a wide variety of astrophysical environments. A potentially important subclass of these materials are PAHs whose edges contain excess H atoms (Hn-PAHs). This type of compound may be present in space, but it has been difficult to assess this possibility because of a lack of suitable laboratory spectra to assist with analysis of astronomical data. We present 4000-500 cm-1 (2.5-20 µm) infrared spectra of 23 Hn-PAHs and related molecules isolated in argon matrices under conditions suitable for interpretation of astronomical data. Spectra of molecules with mixed aromatic and aliphatic domains show characteristics that distinguish them from fully aromatic PAH equivalents. Two major changes occur as PAHs become more hydrogenated: (1) aromatic C-H stretching bands near 3.3 µm weaken and are replaced with stronger aliphatic bands near 3.4 µm, and (2) aromatic C-H out-of-plane bending mode bands in the 11-15 µm region shift and weaken concurrent with growth of a strong aliphatic -CH2-deformation mode near 6.9 µm. Implications for interpreting astronomical spectra are discussed with emphasis on the 3.4 and 6.9 µm features. Laboratory data is compared with emission spectra from IRAS 21282+5050, an object with normal PAH emission features, and IRAS 22272+5435 and IRAS 0496+3429, two protoplanetary nebulae with abnormally large 3.4 µm features. We show that 'normal' PAH emission objects contain relatively few Hn-PAHs in their emitter populations, but less evolved protoplanetary nebulae may contain significant abundances of these molecules.

Nearby early-type galaxies with ionized gas

We present low resolution Spitzer-IRS spectra of 40 ETGs, selected from a sample of 65 ETGs showing emission lines in their optical spectra. We homogeneously extract the mid-infrared (MIR) spectra, and after the proper subtraction of a "passive" ETG template, we derive the intensity of the ionic and molecular lines and of the polycyclic aromatic hydrocarbon emission features. We use MIR diagnostic diagrams to investigate the powering mechanisms of the ionized gas. The mid-infrared spectra of early-type galaxies show a variety of spectral characteristics. We empirically sub-divide the sample into five classes of spectra with common characteristics. Class-0, accounting for 20% of the sample, are purely passive ETGs with neither emission lines nor PAH features. Class-1 show emission lines but no PAH features, and account for 17.5% of the sample. Class-2, in which 50% of the ETGs are found, as well as having emission lines, show PAH features with unusual ratios, e.g. 7.7 {\mu}m/11.3 {\mu}m \leq 2.3. Class-3 objects have emission lines and PAH features with ratios typical of star-forming galaxies. 7.5% of objects fall in this class, likely to be objects in a starburst/post-starburst regime. Class-4, containing only 5% of the ETGs, is dominated by a hot dust continuum. The diagnostic diagram [Ne III]15.55{\mu}m/[Ne II]12.8{\mu}m vs. [S III]33.48{\mu}m/[Si II]34.82{\mu}m, is used to investigate the different mechanisms ionizing the gas. If we exclude NGC 3258 where a starburst seems present, most of our ETGs contain gas ionized via either AGN-like or shock phenomena, or both. Most of the spectra in the present sample are classified as LINERs in the optical window. The proposed MIR spectral classes show unambiguously the manifold of the physical processes and ionization mechanisms, from star formation, low level AGN activity, to shocks, present in LINER nuclei.