Μm Polycyclic Aromatic Hydrocarbon Emission Research Articles

Probing Disk Ice Content and Polycyclic Aromatic Hydrocarbon Emission through Multiband MagAO+Clio Images of HD 141569

We present resolved images of the inner disk component around HD 141569A using the Magellan adaptive optics system with the Clio2 1–5 μm camera, offering a glimpse of a complex system thought to be in a short evolutionary phase between protoplanetary and debris disk stages. We use a reference star along with the Karhunen–Loéve image projection (KLIP) algorithm for point-spread function subtraction to detect the disk inward to about 0.″24 (∼25 au assuming a distance of 111 pc) at high signal-to-noise ratios at L′ (3.8 μm), Ls (3.3 μm), and narrowband Ice (3.1 μm). We identify an arc or spiral arm structure at the southeast extremity, consistent with previous studies. We implement forward modeling with a simple disk model within the framework of a Markov Chain Monte Carlo sampler to better constrain the geometrical attributes and photometry using our KLIP-reduced disk images. We then leverage these modeling results to facilitate a comparison of the measured brightness in each passband to find a reduction in scattered light from the disk in the Ice filter, implying significant absorption due to water ice in the dust. Additionally, our best-fit disk models exhibit peak brightness in the southwestern, back-scattering region of the disk, which we suggest to be possible evidence of 3.3 μm polycyclic aromatic hydrocarbon emission. However, we point out the need for additional observations with bluer filters and more complex modeling to confirm these hypotheses.

New Insights in the Bubble Wall of NGC 3324: Intertwined Substructures and a Bipolar Morphology Uncovered by JWST

We report the discovery of intertwined/entangled substructures toward the bubble wall of NGC 3324 below a physical scale of 4500 au, which is the sharp edge/ionization front/elongated structure traced at the interface between the H ii region and the molecular cloud. The sharp edge appears wavy in the Spitzer 3.6–8.0 μm images (resolution ∼2″). Star formation signatures have mostly been traced on one side of the ionization front, which lies on the molecular cloud’s boundary. The James Webb Space Telescope’s (JWST) near- and mid-infrared images (resolution ∼0.″07—0.″7) are employed to resolve the sharp edge, which has a curvature facing the exciting O-type stars. The elongated structures are associated with the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission, the 4.05 μm ionized emission, and the 4.693 μm H2 emission. However, the PAH-emitting structures are depicted between the other two. The H2 emission reveals numerous intertwined substructures that are not prominently traced in the 3.3 μm PAH emission. The separation between two substructures in the H2 emission is ∼1.″1 or 2420 au. The intertwined substructures are traced in the spatial areas associated with the neutral to H2 transition zone, suggesting the origin of these structures by “thin-shell” instability. Furthermore, an arc-like feature traced in the Spitzer 3.6–8.0 μm images is investigated as a bipolar H ii region (extent ∼0.35 pc) at T d ∼25–28 K using the JWST images. A massive-star candidate VPHAS-OB1 #03518 seems to be responsible for the bipolar H ii region.

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.

Welcome to the Twilight Zone: The Mid-infrared Properties of Post-starburst Galaxies

Abstract We investigate the optical and Wide-field Survey Explorer (WISE) colors of “E+A” identified post-starburst galaxies, including a deep analysis of 190 post-starbursts detected in the 2 μm All Sky Survey Extended Source Catalog. The post-starburst galaxies appear in both the optical green valley and the WISE Infrared Transition Zone. Furthermore, we find that post-starbursts occupy a distinct region of [3.4]–[4.6] versus [4.6]–[12] WISE colors, enabling the identification of this class of transitioning galaxies through the use of broadband photometric criteria alone. We have investigated possible causes for the WISE colors of post-starbursts by constructing a composite spectral energy distribution (SED), finding that the mid-infrared (4–12 μm) properties of post-starbursts are consistent with either 11.3 μm polycyclic aromatic hydrocarbon emission, or thermally pulsating asymptotic giant branch (TP-AGB) and post-AGB stars. The composite SED of extended post-starburst galaxies with 22 μm emission detected with signal-to-noise ratio requires a hot dust component to produce their observed rising mid-infrared SED between 12 and 22 μm. The composite SED of WISE 22 μm non-detections (S/N < 3), created by stacking 22 μm images, is also flat, requiring a hot dust component. The most likely source of the mid-infrared emission of these E+A galaxies is a buried active galactic nucleus (AGN). The inferred upper limits to the Eddington ratios of post-starbursts are 10−2–10−4, with an average of 10−3. This suggests that AGNs are not radiatively dominant in these systems. This could mean that including selections capable of identifying AGNs as part of a search for transitioning and post-starburst galaxies would create a more complete census of the transition pathways taken as a galaxy quenches its star formation.

Polycyclic Aromatic Hydrocarbon Emission in Spitzer/IRS Maps. II. A Direct Link between Band Profiles and the Radiation Field Strength

Abstract We decompose the observed 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission complexes in a large sample of over 7000 mid-infrared spectra of the interstellar medium using spectral cubes observed with the Spitzer/IRS-SL instrument. In order to fit the 7.7 μm PAH emission complex we invoke four Gaussian components, which are found to be very stable in terms of their peak positions and widths across all of our spectra, and subsequently define a decomposition with fixed parameters, which gives an acceptable fit for all the spectra. We see a strong environmental dependence on the interrelationships between our band fluxes—in the H ii regions all four components are intercorrelated, while in the reflection nebulae (RNs) the inner and outer pairs of bands correlate in the same manner as previously seen for NGC 2023. We show that this effect arises because the maps of RNs are dominated by emission from strongly irradiated photodissociation regions, while the much larger maps of H ii regions are dominated by emission from regions much more distant from the exciting stars, leading to subtly different spectral behavior. Further investigation of this dichotomy reveals that the ratio of two of these components (centered at 7.6 and 7.8 μm) is linearly related to the UV-field intensity (log G 0). We find that this relationship does not hold for sources consisting of circumstellar material, which are known to have variable 7.7 μm spectral profiles.

STAR FORMATION IN ULTRALUMINOUS INFRARED GALAXIES PROBED WITH AKARI NEAR-INFRARED SPECTROSCOPY

ABSTRACT We conducted systematic observations of the H i Brα line (4.05 μm) and the polycyclic aromatic hydrocarbon (PAH) feature (3.3 μm) in 50 nearby ( ) ultraluminous infrared galaxies (ULIRGs) with AKARI. The Brα line is predicted to be the brightest among the H i lines under conditions of high dust extinction ( mag). The Brα line traces ionizing photons from OB stars and so is used as an indicator of star formation on the assumption of the initial mass function. We detected the Brα line in 33 ULIRGs. The luminosity of the line ( ) correlates well with that of the 3.3 μm PAH emission ( ). Thus we utilize as an indicator of star formation in fainter objects where the Brα line is undetected. The mean ratio in LINERs/Seyferts is significantly lower than that in H ii galaxies. This difference is reconfirmed with the ratio in the larger sample (46 galaxies). Using the ratios, we estimate that the contribution of starburst in LINERs/Seyferts is , and active galactic nuclei contribute the remaining . However, comparing the number of ionizing photons, , derived from with that, Q IR, expected from the star formation rate required to explain , we find that the mean ratio is only (55.5 ± 7.5)% even in H ii galaxies, which are thought to be energized by pure starburst. This deficit of ionizing photons traced by the Brα line is significant even taking heavy dust extinction into consideration. We propose that dust within H ii regions absorbs a significant fraction of ionizing photons.

AKARIIRC INFRARED 2.5-5 μm SPECTROSCOPY OF A LARGE SAMPLE OF LUMINOUS INFRARED GALAXIES

We present the results of our systematic infrared 2.5–5 μm spectroscopy of 60 luminous infrared galaxies (LIRGs) with infrared luminosities LIR = 1011–1012 L☉ and 54 ultraluminous infrared galaxies (ULIRGs) with LIR ⩾ 1012 L☉, using the AKARI Infrared Camera (IRC). AKARI IRC slit-less spectroscopy allows us to probe the full range of emission from these galaxies, including spatially extended components. The 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission features, hydrogen recombination emission lines, and various absorption features are detected and used to investigate the properties of these galaxies. Because of the relatively small effect of dust extinction in the infrared range, quantitative discussion of these dusty galaxy populations is possible. For sources with clearly detectable Brβ (2.63 μm) and Brα (4.05 μm) emission lines, the flux ratios are found to be similar to those predicted by case B theory. Starburst luminosities are estimated from both 3.3 μm PAH and Brα emission, which roughly agree with each other. In addition to the detected starburst activity, a significant fraction of the observed sources display signatures of obscured active galactic nuclei (AGNs), such as low PAH equivalent widths, large optical depths of dust absorption features, and red continuum emission. The energetic importance of optically elusive buried AGNs in optically non-Seyfert galaxies tends to increase with increasing galaxy infrared luminosity, from LIRGs to ULIRGs.

The near- and far-infrared colours of MASH planetary nebulae

We have analysed the near-infrared (NIR) and far-infrared (FIR) colours of MASH I and MASH II (the Macquarie/AAO/Strasbourg surveys) planetary nebulae (PNe), using data deriving from the Two-Micron All-Sky Survey and Infrared Astronomical Satellite .W e were able to identify ∼5 per cent of the sources in the NIR, and a slightly larger fraction (∼12 per cent) in the FIR. It is concluded that whilst the NIR colours of these nebulae are consistent with those of less evolved (and higher surface brightness) PNe, their FIR colours are markedly different. This disparity is likely to arise as a result of an evolution in dust temperatures, in their line emission characteristics, and in the relative contributions of the 8.6 and 11.3 μm polycyclic aromatic hydrocarbon emission features. A rump of ∼9 per cent of the detected sources have values log[F(25 μm)/F(60 μm)] which are lower than can be explained in terms of normal nebular evolution, however. If these are comparable in nature to the undetected PNe, then this would argue that ∼1 in 10 of MASH I and II nebulae may represent galactic H II regions, Stromgren spheres, symbiotic nebulae or other unrelated categories of source.

The Most Luminous Starbursts in the Universe

A summary of starburst luminosities based on polycyclic aromatic hydrocarbon (PAH) features is given for 243 starburst galaxies with 0 < z < 2.5, observed with the Spitzer Infrared Spectrograph. Luminosity ν Lν(7.7 μm) for the peak luminosity of the 7.7 μm PAH emission feature is found to scale as log [ ν Lν(7.7 μm) ] = 44.63(± 0.09) + 2.48(± 0.28) log (1 + z) for the most luminous starbursts observed. Empirical calibrations of ν Lν(7.7 μm) are used to determine bolometric luminosity LIR and the star formation rate (SFR) for these starbursts. The most luminous starbursts found in this sample have log LIR = 45.4(± 0.3) + 2.5(± 0.3) log (1 + z) , in ergs s−1, and the maximum star formation rates for starbursts in units of M☉ yr−1 are log (SFR) = 2.1(± 0.3) + 2.5(± 0.3) log (1 + z) , up to z = 2.5. The exponent for pure luminosity evolution agrees with optical and radio studies of starbursts but is flatter than previous results based in infrared source counts. The maximum star formation rates are similar to the maxima determined for submillimeter galaxies; the most luminous individual starburst included within the sample has log LIR = 46.9, which gives an SFR = 3.4 × 103 M☉ yr−1.

Infrared 3-4 μm Spectroscopy of Infrared Luminous Galaxies with Possible Signatures of Obscured Active Galactic Nuclei

We present the results of infrared 2.8-4.1 μm (L-band) spectroscopy of nearby infrared luminous galaxies with possible signatures of dust-obscured active galactic nuclei (AGNs) in data at other wavelengths. The samples are chosen to include sources with a radio excess relative to far-infrared emission, strong absorption features in mid-infrared 5-11.5 μm spectra, unusually weak [C II] 158 μm emission relative to the far-infrared continuum, and radio galaxies classified optically as narrow-line objects. Our aim is to investigate whether the signatures of possible obscured AGNs can be detected in our L-band spectra based on the strengths of emission and absorption features. Six of nine observed sources clearly show 3.3 μm polycyclic aromatic hydrocarbon emission features, a good starburst indicator. An absorption feature at 3.1 μm due to ice-covered dust is detected in IRAS 04154+1755 and IRAS 17208-0014. The signature of a bare carbonaceous dust absorption feature at 3.4 μm is seen in NGC 1377. Our L-band spectra reveal strong signatures of obscured AGNs in all three optical Seyfert 2 galaxies (IRAS 04154+1755, Cygnus A, and 3C 234) and two galaxies classified optically as non-Seyfert galaxies (NGC 828 and NGC 1377). Among the remaining optical non-Seyfert galaxies, IRAS 17208-0014 might also show a buried AGN signature, whereas no explicit AGN evidence is seen in the L-band spectra of the mid-infrared absorption feature source IRAS 15250+3609 and two weak [C II] emitters, IC 860 and CGCG 1510.8+0725.

Infrared 3–4 μm Spectroscopic Investigations of a Large Sample of Nearby Ultraluminous Infrared Galaxies

We present infrared L-band (3-4 μm) nuclear spectra of a large sample of nearby ultraluminous infrared galaxies (ULIRGs). ULIRGs classified optically as non-Seyfert galaxies (LINERs, H II regions, and unclassified) are our main targets. Using the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission and absorption features at 3.1 μm due to ice-covered dust and at 3.4 μm produced by bare carbonaceous dust, we search for signatures of powerful AGNs deeply buried along virtually all lines of sight. The 3.3 μm PAH emission, the signatures of starbursts, is detected in all but two non-Seyfert ULIRGs, but the estimated starburst magnitudes can account for only a small fraction of the infrared luminosities. Three LINER ULIRGs show spectra typical of almost pure buried AGNs, namely, strong absorption features with very small equivalent width PAH emission. Besides these three sources, 14 LINER and three H II ULIRGs' nuclei show strong absorption features whose absolute optical depths suggest an energy source more centrally concentrated than the surrounding dust, such as a buried AGN. In total, 17 out of 27 (63%) LINER and 3 out of 13 (23%) H II ULIRGs' nuclei show some degree of evidence for powerful buried AGNs, suggesting that powerful buried AGNs may be more common in LINER ULIRGs than in H II ULIRGs. The evidence of AGNs is found in non-Seyfert ULIRGs with both warm and cool far-infrared colors. These spectra are compared with those of 15 ULIRGs' nuclei with optical Seyfert signatures taken for comparison. The overall spectral properties suggest that the total amount of dust around buried AGNs in non-Seyfert ULIRGs is systematically larger than that around AGNs in Seyfert 2 ULIRGs. We argue that the optical (non)detectability of Seyfert signatures in ULIRGs is highly dependent on how deeply buried the AGNs are, and that it is essential to properly evaluate the energetic importance of buried AGNs in non-Seyfert ULIRGs.

Obscured Star Formation in the Central Region of the Dwarf Galaxy NGC 5253

We present Hubble Space Telescope (HST) NICMOS observations (1.1-2.2 μm) and 1.9-4.1 μm spectroscopy of the central region of the dwarf galaxy NGC 5253. The HST NICMOS observations reveal the presence of a nuclear double star cluster separated by 03-04, or 6-8 pc (for a distance d = 4.1 Mpc). The double star cluster, also a bright double source of Paα emission, appears to be coincident with the double radio nebula detected at 1.3 cm. The eastern near-infrared star cluster (C1) is identified with the youngest optical cluster, whereas the western star cluster (C2), although it is almost completely obscured in the optical, becomes the brightest star cluster in the central region of NGC 5253 at wavelengths longer than 2 μm. Both clusters are extremely young, with ages of approximately 3.5 Myr. C2 is more massive than C1 by a factor of 6-20 (MC2 = 7.7 × 105-2.6 × 106 M☉, for a Salpeter initial mass function [IMF] in the mass range 0.1-100 M☉). Analysis of the circumnuclear spectrum excluding C1 and C2, as well as of a number of other near-infrared-selected clusters with a range of (young) ages, suggests that the star formation was triggered across the central regions of the galaxy. We have also modeled the nuclear UV to mid-infrared spectral energy distribution (SED) of NGC 5253 and found that the infrared part is well modeled by a highly obscured (AV = 17 mag) young starburst with a stellar mass consistent with our photometric estimates for C1 and C2. The SED model predicts a moderately bright polycyclic aromatic hydrocarbon (PAH) feature at 3.3 μm that is not detected in our nuclear L-band spectrum. NGC 5253's low metallicity and a top-heavy IMF likely combine to suppress the 3.3 μm PAH emission that is commonly seen in more massive starburst systems.

Compact Nuclear Starbursts in Seyfert 2 Galaxies from the CfA and 12 Micron Samples

We present infrared 2.8-4.1 μm slit spectra of 32 Seyfert 2 galaxies in the CfA and 12 μm samples. The 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission feature was used to estimate the absolute magnitude of a compact nuclear starburst (less than a few hundred parsecs in size) that is presumed to have occurred in the outer region of an obscuring dusty molecular torus around a central supermassive black hole. We detected 3.3 μm PAH emission in 11 of the 32 Seyfert 2 nuclei in our sample, providing evidence for the presence of compact nuclear starbursts in a significant fraction of Seyfert 2 nuclei. However, the rest-frame equivalent widths of the 3.3 μm PAH emission and the 3.3 μm PAH-to-infrared luminosity ratios measured in this study suggest that compact nuclear starbursts generally do not contribute significantly to the observed 3-4 μm nuclear fluxes or to the infrared luminosities of Seyfert 2 galaxies. Absorption features at 3.4 μm from bare dust were clearly detected in only two of the nuclei, and features at 3.1 μm from ice-covered dust were detected in only one nucleus. If the dust properties in the direction of these Seyfert 2 nuclei do not differ significantly from the Galactic interstellar medium, then these small absorption optical depths suggest that dust extinction toward the 3-4 μm continuum emitting region in the innermost part of the obscuring dusty torus is modest: AV < 50-60 mag. Finally, the 3.3 μm PAH emission luminosities measured in this study were found to be significantly correlated with IRAS 12 and 25 μm and nuclear N-band (10.6 μm) luminosities. If these three luminosities trace the power of the active galactic nucleus (AGN), then the luminosities of compact nuclear starbursts and AGNs are correlated. This correlation is in agreement with theories predicting that the presence of a compact nuclear starburst in the torus leads to an enhancement of the mass accretion rate onto the central supermassive black hole.

The 3.4-μm absorption feature towards three obscured active galactic nuclei

The results of 3–4 µm spectroscopy towards the nuclei of NGC 3094, NGC 7172, and NGC 7479 are reported. In ground-based 8–13 µm spectra, all the sources have strong absorption-like features at �10 µm, but they do not have detectable polycyclic aromatic hydrocarbon (PAH) emission features. The 3.4 µm carbonaceous dust absorption features are detected towards all nuclei. NGC 3094 shows a detectable 3.3 µm PAH emission feature, while NGC 7172 and NGC 7479 do not. Nuclear emission whose spectrum shows dust absorption features but no PAH emission features is thought to dominated by highly obscured active galactic nuclei (AGNs) activity. For NGC 7172, NGC 7479, and three other such nuclei in the literature, we investigate the optical depth ratios between the 3.4 µm carbonaceous dust and 9.7 µm silicate dust absorption (�3.4/�9.7). The �3.4/�9.7 ratios towards three highly obscured AGNs with face-on host galaxies are systematically larger than the ratios in the Galactic diffuse interstellar medium or the ratios for two highly obscured AGNs with edge-on host galaxies. We suggest that the larger ratios can be explained if the obscuring dust is so close to the central AGNs that a temperature gradient occurs in it. If this idea is correct, our results may provide spectroscopic evidence for the presence of the putative “dusty tori” in the close vicinity of AGNs.

3–4 Micron Spectroscopy of Seyfert 2 Nuclei to Quantitatively Assess the Energetic Importance of Compact Nuclear Starbursts

We report on 3-4 μm slit spectroscopy of 13 Seyfert 2 nuclei. The 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission is used to estimate the magnitudes of compact nuclear starbursts (on scales less than a few hundred parsecs) and to resolve the controversy over their energetic importance in Seyfert 2 nuclei. For three selected Seyfert 2 nuclei that have been well studied in the UV, the magnitudes of the compact nuclear starbursts estimated from the 3.3 μm PAH emission (with no extinction correction) are in satisfactory quantitative agreement with those based on the UV after extinction correction. This agreement indicates that the flux attenuation of compact nuclear starburst emission due to dust extinction is insignificant at 3-4 μm, and thus allows us to use the observed 3.3 μm PAH luminosity to estimate the magnitudes of the compact nuclear starbursts in Seyfert 2 nuclei. Based directly on our 3-4 μm slit spectra, the following two main conclusions are drawn: (1) except in one case, the observed nuclear 3-4 μm emission is dominated by active galactic nuclei (AGNs) and not by starbursts, and (2) compact nuclear starbursts are detected in 6 out of 13 Seyfert 2 nuclei, but cannot dominate the energetics of the galactic infrared dust emission in the majority of the observed Seyfert 2 galaxies. For several sources for which Infrared Space Observatory spectra taken with larger apertures and/or soft X-ray data are available, these data are combined with our 3-4 μm slit spectra, and it is suggested that (3) extended (kpc scale) star formation activity is energetically more important than compact nuclear starbursts, and contributes significantly to the infrared luminosities of Seyfert 2 galaxies, although the AGN is still an important contributor to the luminosities, and (4) the bulk of the energetically significant extended star formation activity is of starburst type rather than quiescent normal disk star formation; the extended starbursts are responsible for the superwind-driven soft X-ray emission from Seyfert 2 galaxies. Finally, a correlation between the luminosities of AGNs and compact nuclear starbursts is implied; more powerful AGNs tend to be related to more powerful compact nuclear starbursts.

NGC 7027: New 7.8–20.0 μm Array Camera and Hα/Hβ CCD Image Analysis of Dust, PAH and Ionized Gas Distribution

Infrared array camera images of the planetary nebula NGC7027 at seven wavelenghts between 7.8 and 20μm taken at the IRTF telescope are compared to Hα and Hβ images obtained at the CFHT, and published 2-, 6-, and 20-cm VLA maps, all made with better than ∼ 1 arcsec resolution. The mid-infrared images have also been deconvolved using the Maximum Residual Likelihood algorithm to show structural details on the ∼ 0.3 arcsec scale. The visible, infrared and radio images are aligned with ±0.2 arcsec astrometric precision using common features and visible stars. Image algebra can then be readily performed. The images are analysed to determine the spatial distribution of poly cyclic aromatic hydrocarbon (PAH) grains and ionized gas relative to the continuum dust emission. The results show the extent of the mid-infrared dust continuum, the PAH emission features at 8.65 and 11.25μm, and would reveal any contribution from 9.8 μm “silicate” emission. The optically thicker 20-cm emission more closely resembles the extinction-corrected Hα distribution. The large grains giving rise to the visible extinction (derived from the Hα/Hβ ratio) are distribued differently from the mid-infrared and ionized gaz, indicating the location of the outer cold dust. The 11.6μm image could contain both PAH and continuum grain emission, and we have attempted to separate the two components. It appears that the overall emission distributions are quite similar for all the infrared wavelengths, so the various dust constituents are generally uniformly mixed. However there are indications that the 11.6μm PAH emission distribution is patchy and not identical to the continuum dust, although not distinctly different in radial distance from the center. The ionized gas distribution peaks inside that of the dust continuum. The visible/infrared/radio spatial analysis, and color indices calculated from the infrared images, are used to develop a detailed new morphological model of NGC7027 based on this comprehensive multi-wavelength data set.