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

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.

We present James Webb Space Telescope (JWST) Near Infrared Spectrograph (NIRSpec) and Mid-infrared Instrument integral field spectroscopy of the nearby blue compact dwarf II Zw 40, which has a low metallicity of 25% of solar. Leveraging the high spatial/spectral resolution and wavelength coverage of JWST/NIRSpec, we present robust detections of the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission on 20 pc scales. The strength of the Pfδ emission relative to the 3.3 PAH feature is significantly stronger than typical higher-metallicity star-forming galaxies. We find that 3.3 μm PAH emission is concentrated near the northern super star cluster and is cospatial with CO gas. A strong correlation exists between the 3.3/11.3 PAH ratio and radiation hardness probed by [Ne iii]/[Ne ii], providing evidence of photodestruction of PAH molecules in intense radiation environments. Our analysis shows that while the overall PAH fraction is lower in II Zw 40 than in higher-metallicity galaxies, the contribution of the 3.3 μm PAH feature to the total PAH emission is higher. We propose that the PAH size distribution is fundamentally shaped by two competing mechanisms in low-metallicity environments: photodestruction and inhibited growth. Additionally, the high radiation field intensity in II Zw 40 suggests that multiphoton heating of PAHs may be an important effect. As one of the first spatially resolved studies of aromatic emission in a low-metallicity environment, our spectroscopic results offer practical guidance for future observations of the 3.3 μm PAH feature in low-metallicity galaxies using JWST.

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.

The earliest stages of star and cluster formation are hidden within dense cocoons of gas and dust, limiting their detection at optical wavelengths. With the unprecedented infrared capabilities of JWST, we can now observe dust-enshrouded star formation with ∼10 pc resolution out to ∼20 Mpc. Early findings from PHANGS-JWST suggest that 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission can identify star clusters in their dust-embedded phases. Here, we extend this analysis to 19 galaxies from the PHANGS-JWST Cycle 1 Treasury survey, providing the first characterization of compact sources exhibiting 3.3 μm PAH emission across a diverse sample of nearby star-forming galaxies. We establish a selection criteria based on a median color threshold of F300M − F335M = 0.67 at F335M = 20 and identify 1816 sources. These sources are predominantly located in dust lanes, spiral arms, rings, and galaxy centers, with ∼87% showing concentration indices (CIs) similar to optically detected star clusters. Comparison with the PHANGS-HST catalogs suggests that PAH emission fades within ∼3 Myr. The Hα equivalent width of PAH emitters is 1–2.8 times higher than that of young PHANGS-HST clusters, providing evidence that PAH emitters are on average younger. Analysis of the bright portions of luminosity functions (which should not suffer from incompleteness) shows that young dusty clusters may increase the number of optically visible ≤3 Myr old clusters in PHANGS-HST by a factor between ∼1.8× and 8.5×.

We report on the results of systematic infrared 3–4 μm spectroscopy of 23 Seyfert 1 and 32 Seyfert 2 galaxies. The putative nuclear starbursts in dusty tori of Seyfert galaxies are investigated through the 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission feature. The 3.3 μm PAH emission is detected in roughly a half of the observed Seyfert galaxies. Since dust extinction is insignificant at infrared 3–4 μm, we can quantitatively estimate the magnitudes of the nuclear starbursts from the observed 3.3 μm PAH emission luminosities. Figure 1 compares the luminosities of nuclear starbursts and AGNs. We find that (1) nuclear starburst to AGN luminosity ratios are similar between Seyfert 1 and 2 galaxies, and that (2) nuclear starburst luminosity positively correlates with AGN power in both types of Seyfert galaxies [1],[2]. Our results suggest that nuclear starbursts are physically closely related to AGNs and control the mass accretion rate onto the central AGNs, through the increased turbulence of molecular gas in the tori and/or increased radiation effects.

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.

The 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.

We present maps of ionized gas (traced by Paα and Brα) and 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission in the nearby spiral galaxy NGC 628, derived from new JWST/NIRCam data from the Feedback in Emerging extrAgalactic Star clusTers (FEAST) survey. With this data, we investigate and calibrate the relation between 3.3 μm PAH emission and star formation rate (SFR) in and around emerging young star clusters (eYSCs) on a scale of ∼40 pc. We find a tight (correlation coefficient ρ ∼ 0.9) sublinear (power-law exponent α ∼ 0.75) relation between the 3.3 μm PAH luminosity surface density and SFR traced by Brα for compact, cospatial (within 0.″16 or ∼7 pc) peaks in Paα, Brα, and 3.3 μm (eYSC–I). The scatter in the relationship does not correlate well with variations in local interstellar medium metallicity, due to a radial metallicity gradient, but rather is likely due to stochastic sampling of the stellar initial mass function (IMF) and variations in the PAH heating and age of our sources. The deviation from a linear relation may be explained by PAH destruction in more intense ionizing environments, variations in age, and IMF stochasticity at intermediate to low luminosities. We test our results with various continuum subtraction techniques using combinations of NIRCam bands and find that they remain robust with only minor differences in the derived slope and intercept. An unexpected discrepancy is identified between the relations of hydrogen recombination lines (Paα versus Brα; Hα versus Brα).

We present the result of a systematic infrared 2.5-5 um spectroscopic study of 22 nearby infrared galaxies over a wide infrared luminosity range (10 < log(L_IR / Lsun) < 13) obtained from AKARI Infrared Camera (IRC). The unique band of the AKARI IRC spectroscopy enables us to access both the 3.3 um polycyclic aromatic hydrocarbon (PAH) emission feature from star forming activity and the continuum of torus-dust emission heated by an active galactic nucleus (AGN). Applying our AGN diagnostics to the AKARI spectra, we discover 14 buried AGNs. The large fraction of buried AGNs suggests that AGN activity behind the dust is almost ubiquitous in ultra-/luminous infrared galaxies (U/LIRGs). We also find that both the fraction and energy contribution of buried AGNs increase with infrared luminosity from 10 < log(L_IR / Lsun) < 13, including normal infrared galaxies with log (L_IR / Lsun) < 11. The energy contribution from AGNs in the total infrared luminosity is only ~7% in LIRGs and ~20% in ULIRGs, suggesting that the majority of the infrared luminosity originates from starburst activity. Using the PAH emission, we investigate the luminosity relation between star formation and AGN. We find that these infrared galaxies exhibit higher star formation rates than optically selected Seyfert galaxies with the same AGN luminosities, implying that infrared galaxies could be an early evolutionary phase of AGN.

We present a low-resolution mid-infrared spectroscopic survey of an unbiased sample of 62 ultraluminous infrared galaxies (ULIRGs) (LIR > 1012L⊙, z ≤ 0.3) using ISOPHOT-S on board the Infrared Space Observatory (ISO). For comparison, we also present ISOPHOT-S spectra for 23 active galactic nuclei (AGNs) and 15 starburst and normal galaxies. The line-to-continuum ratio of the 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission feature is used as a discriminator between starburst and AGN activity in ULIRGs. We find that the majority of ULIRGs are predominantly powered by starbursts. The ratio of PAH over infrared luminosities, LPAH/LIR, for starburst-dominated ULIRGs is very similar to the ratio found for template starbursts. The shapes of the PAH features are sometimes unusual. Extinction has a noticeable effect on the PAH spectra of ULIRG starbursts. We have obtained high-resolution near-infrared imaging for the southern ISOPHOT-S ULIRGs in order to investigate their evolution stage. The majority (68%) of the ULIRGs imaged are double systems, and all show distorted morphologies. Of the 23 double-nuclei systems, 17 appear at linear separations between 4 and 14 kpc, with a mean separation of 6.5 kpc. Using the separations measured from our new near-infrared imaging as well as from the literature, we have examined whether ULIRGs that are advanced mergers are more AGN-like. We have found no such evidence, contrary to what is postulated by the classical evolutionary scenario. No correlation is found between the stage of merger in ULIRGs and their infrared luminosity. In fact, we find that systems in the early stages of merging may well put out maximum luminosity. We also find that the total mass of interstellar gas, as estimated from the CO (1 → 0) luminosity, does not decrease with decreasing merger separation. When both an AGN and a starburst occur concurrently in ULIRGs, we find that the starburst dominates the luminosity output. We propose that the available gas reservoir and the individual structure of the interacting galaxies plays a major role in the evolution of the system.

Together with gas, stars, and supermassive black holes, dust is crucial in stellar and galaxy evolution. Hence, understanding galaxies’ dust properties across cosmic time is critical to studying their evolution. In addition to photometric constraints on the absorption of blue light and its reemission at infrared wavelengths, dust grain properties can be explored spectroscopically via polycyclic aromatic hydrocarbon (PAH) emission bands in the mid-IR. The new SPHEREx space telescope conducts an all-sky spectrophotometric survey of stars and galaxies at wavelengths of 0.75–5 μm, making it ideal for studying the widespread presence of the 3.3 μm PAH emission across galaxy populations out to z ∼ 0.4. In this paper, we simulated galaxy spectra to investigate SPHEREx's capability to study PAH emission in such galaxies. We find that for the all-sky survey the PAH 3.3 μm emission band flux can be measured to 30% accuracy at log ( M / M ⊙ ) &gt; 9.5 and star formation rate (SFR) &gt; 1 M ⊙ yr−1 at z = 0.1, log ( M / M ⊙ ) &gt; 10.5 and SFR &gt; 10 M ⊙ yr−1 at z = 0.2–0.3, and log ( M / M ⊙ ) &gt; 11 and SFR &gt; 100 M ⊙ yr−1 at z = 0.4. For deep SPHEREx fields, a factor of ∼10 deeper sensitivity limits can be reached. Overall, SPHEREx will enable the measurement of the 3.3 μm PAH band emission in several hundred thousand galaxies across the sky, providing a population study of the smallest dust grains (“nano grains”) and radiation properties in massive galaxies in the nearby Universe.

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.

We report on infrared K- (2-2.5 ?m) and L-band (2.8-4.1 ?m) slit spectroscopy of 23 Seyfert 1 galaxies in the CfA and 12 ?m samples. A polycyclic aromatic hydrocarbon (PAH) emission feature at 3.3 ?m in the L band is primarily used to investigate nuclear star-forming activity in these galaxies. The 3.3 ?m PAH emission is detected in 10 sources (=43%), demonstrating that detection of nuclear star formation in a significant fraction of Seyfert 1 galaxies is now feasible. For the PAH-detected nuclei, the surface brightness values of the PAH emission are as high as those of typical starbursts, suggesting that the PAH emission probes the putative nuclear starbursts in the dusty tori around the central active galactic nuclei (AGNs). The magnitudes of the nuclear starbursts are quantitatively estimated from the observed 3.3 ?m PAH emission luminosities. The estimated starburst luminosities relative to some indicators of AGN powers in these Seyfert 1 galaxies are compared with 32 Seyfert 2 galaxies in the same samples that we have previously observed. We find that there is no significant difference in nuclear starburst to AGN luminosity ratios of Seyfert 1 and 2 galaxies and that nuclear starburst luminosity positively correlates with AGN power in both types. Our results favor a slightly modified AGN unification model, which predicts that nuclear starbursts occurring in the dusty tori of Seyfert galaxies are physically connected to the central AGNs, rather than the classical unification paradigm, in which the dusty tori simply hide the central AGNs of Seyfert 2 galaxies and reprocess AGN radiation as infrared dust emission in Seyfert galaxies. No significant differences in nuclear star formation properties are recognizable between Seyfert 1 galaxies in the CfA and 12 ?m samples.

Combining Atacama Large Millimeter/submillimeter Array CO(2–1) mapping and JWST near- and mid-infrared imaging, we characterize the relationship between CO(2–1) and polycyclic aromatic hydrocarbon (PAH) emission at ≈100 pc resolution in 70 nearby star-forming galaxies. Leveraging a new Cycle 2 JWST Treasury program targeting nearby galaxies, we expand the sample size by more than an order of magnitude compared to previous ≈100 pc resolution CO–PAH comparisons. In regions of galaxies where most of the gas is likely to be molecular, we find strong correlations between CO(2–1) and 3.3 μm, 7.7 μm, and 11.3 μm PAH emission, estimated from JWST’s F335M, F770W, and F1130W filters. We derive power-law relations between CO(2–1) and PAH emission, with indices in the range 0.8–1.3, implying relatively weak variations in the observed CO-to-PAH ratios across our sample. We find that CO-to-PAH ratios and scaling relationships near H ii regions are similar to those in diffuse sight lines. The main difference between the two types of regions is that sight lines near H ii regions show higher intensities in all tracers. Galaxy centers show higher overall intensities and enhanced CO-to-PAH ratios compared to galaxy disks. Individual galaxies show 0.19 dex scatter in the normalization of CO at fixed I PAH, and this normalization anticorrelates with specific star formation rate and correlates with stellar mass. We provide a prescription that accounts for galaxy-to-galaxy variations, representing our best current empirical predictor to estimate CO(2–1) intensity from PAH emission, allowing one to take advantage of JWST’s excellent sensitivity and resolution to trace cold gas.