Strong Polycyclic Aromatic Hydrocarbon Emission Research Articles

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.

Characterizing spatial variations of PAH emission in the reflection nebula NGC 1333

ABSTRACT Infrared emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 µm, attributed to polycyclic aromatic hydrocarbons (PAHs), show variations in relative intensity, shape, and peak position. These variations depend on the physical conditions of the photodissociation region (PDR) in which strong PAH emission arises but their relationship has yet to be fully quantified. We aim to better calibrate the response of PAH species to their environment using observations with matching apertures and spatial resolution. We present observations from the Field-Imaging Far-Infrared Line Spectrometer onboard the Stratospheric Observatory for Infrared Astronomy of the gas cooling lines [O i] 63, 146 µm and [C ii] 158 µm in the reflection nebula NGC 1333 and use archival dust continuum observations from the Photodetector Array Camera and Spectrometer (PACS) onboard Herschel. We employ PDR modelling to derive the physical conditions and compare these with the characteristics of the PAH emission as observed with the Infrared Spectrometer onboard Spitzer. We find distinct spatial characteristics for the various PAH spectral components. We conclude that the ionic bands (6.2, 7.7, 8.6, and 11.0) and the 7–9 µm emission are due to multiple PAH subpopulations and that the plateaus are distinct from the features perched on top. The 6–9 µm PAH emission exhibits a significant change in behaviour between the irradiated PDR and diffuse outskirts, confirming these bands arise from multiple PAH subpopulations with different underlying molecular properties. We find multiple promising relationships between PAH ratios and the far-ultraviolet radiation field strength but no clear correlations with the PAH ionization parameter.

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.

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.

A TALE OF THREE GALAXIES: ANOMALOUS DUST PROPERTIES IN IRAS F10398+1455, IRAS F21013–0739, AND SDSS J0808+3948

On a galactic scale the 9.7um silicate emission is usually only seen in type 1 active galactic nuclei (AGNs). They usually also display a flat emission continuum at ~5--8um and the absence of polycyclic aromatic hydrocarbon (PAH) emission bands. In contrast, starburst galaxies, luminous infrared (IR) galaxies (LIRGs), and ultraluminous IR galaxies (ULIRGs) exhibit a red 5--8um emission continuum, strong 10um and 18um silicate absorption features, and strong PAH emission bands. Here we report the detection of anomalous dust properties by Spitzer/Infrared Spectrograph in three galaxies (IRASF10398+1455, IRASF21013-0739 and SDSSJ0808+3948) which are characterized by the simultaneous detection of a red 5--8um emission continuum, the 9.7 and 18um silicate emission features as well as strong PAH emission bands. These apparently contradictory dust IR emission properties are discussed in terms of iron-poor silicate composition, carbon dust deficit, small grain size and low dust temperature in the young AGN phase of these three galaxies.

Properties of the high-mass star forming region IRAS 22475+5939

IRAS 22475+5939 has been well studied by previous astronomers, but we can still discover new characteristics about it, using the first observations of the lines from CO J=2−1, 13CO J=2−1 and 13CO J=3−2 by the KOSMA 3m telescope. The mapping of the intensity ratio of 13CO J=3−2 and 13CO J=2−1 shows that the distribution of the temperature has two peaks, neither of which coincide with the IRAS 22475+5939 source or the center of the HII region, but rather are located at the edge of the HII region. Overlays of the Spitzer IRAC 8 μm with CO contours indicate that they are associated with each other and that the strongest polycyclic aromatic hydrocarbon (PAH) emission is at the IRAS 22475+5939 source position. The IRAS LRS spectrum at 7 μm ∼ 23 μm and the PHT-s spectrum at 2 μm ∼ 12 μm of the IRAS 22475+5939 source also exhibit strong PAH emission characteristics in the main PAH bands. The diversity of the PAH family should be responsible for the plateaus in the PAH emission in the PHT-s and IRAS-LRS spectra. Analysis and modeling in the infrared bands suggest that IRAS 22475+5939 is more likely to be a class I young stellar object. Where this is the case, the star is likely to have a temperature of Teff ∼ 9995.8K, mass ∼ 15.34 M⊙, luminosity ∼ 1.54 × 104 L⊙ and age ∼ 1.54 × 104 yr. The model shows that circumstellar disc emission is important for wavelengths between 1 and 10 μm, otherwise, the envelope fluctuates for λ > 10μm. Bipolar outflow is confirmed in the molecular cloud, and the excited star of the HII region could be the driving source of the outflow. High resolution is required for better results.

HUBBLE SPACE TELESCOPEMORPHOLOGIES OFz∼ 2 DUST-OBSCURED GALAXIES. II. BUMP SOURCES

We present Hubble Space Telescope (HST) imaging of 22 ultra-luminous infrared galaxies (ULIRGs) at z~2 with extremely red R-[24] colors (called dust-obscured galaxies, or DOGs) which have a local maximum in their spectral energy distribution (SED) at rest-frame 1.6um associated with stellar emission. These sources, which we call "bump DOGs", have star-formation rates of 400-4000 Msun/yr and have redshifts derived from mid-IR spectra which show strong polycyclic aromatic hydrocarbon emission --- a sign of vigorous on-going star-formation. Using a uniform morphological analysis, we look for quantifiable differences between bump DOGs, power-law DOGs (Spitzer-selected ULIRGs with mid-IR SEDs dominated by a power-law and spectral features that are more typical of obscured active galactic nuclei than starbursts), sub-millimeter selected galaxies (SMGs), and other less-reddened ULIRGs from the Spitzer extragalactic First Look Survey (XFLS). Bump DOGs are larger than power-law DOGs (median Petrosian radius of 8.4 +/- 2.7 kpc vs. 5.5 +/- 2.3 kpc) and exhibit more diffuse and irregular morphologies (median M_20 of -1.08 +/- 0.05 vs. -1.48 +/- 0.05). These trends are qualitatively consistent with expectations from simulations of major mergers in which merging systems during the peak star-formation rate period evolve from M_20 = -1.0 to M_20 = -1.7. Less obscured ULIRGs (i.e., non-DOGs) tend to have more regular, centrally peaked, single-object morphologies rather than diffuse and irregular morphologies. This distinction in morphologies may imply that less obscured ULIRGs sample the merger near the end of the peak star-formation rate period. Alternatively, it may indicate that the intense star-formation in these less-obscured ULIRGs is not the result of a recent major merger.

SPITZER'S VIEW ON AROMATIC AND ALIPHATIC HYDROCARBON EMISSION IN HERBIG Ae STARS

The chemistry of astronomical hydrocarbons, responsible for the well-known infrared emission features detected in a wide variety of targets, remains enigmatic. Here we focus on the group of young intermediate-mass Herbig Ae stars. We have analyzed the aliphatic and polycyclic aromatic hydrocarbon (PAH) emission features in the infrared spectra of a sample of 53 Herbig Ae stars, obtained with the Infrared Spectrograph aboard the Spitzer Space Telescope. We confirm that the PAH-to-stellar luminosity ratio is higher in targets with a flared dust disk. However, a few sources with a flattened dust disk still show relatively strong PAH emission. Since PAH molecules trace the gas disk, this indicates that gas disks may still be flared, while the dust disk has settled due to grain growth. There are indications that the strength of the 11.3 μm feature also depends on dust disk structure, with flattened disks being less bright in this feature. We confirm that the CC bond features at 6.2 and 7.8 μm shift to redder wavelengths with decreasing stellar effective temperature. Moreover, we show that this redshift is accompanied by a relative increase of aliphatic CH emission and a decrease of the aromatic 8.6 μm CH feature strength. Cool stars in our sample are surrounded by hydrocarbons with a high aliphatic/aromatic CH ratio and a low aromatic CH/CC ratio, and vice versa for the hot stars. We conclude that, while the overall hydrocarbon emission strength depends on the dust disk's geometry, the relative differences seen in the IR emission features in disks around Herbig Ae stars are mainly due to chemical differences of the hydrocarbon molecules induced by the stellar UV field. Strong UV flux reduces the aliphatic component and emphasizes the spectral signature of the aromatic molecules in the IR spectra.

DUST ABUNDANCE AND PROPERTIES IN THE NEARBY DWARF GALAXIES NGC 147 AND NGC 185

We present new mid- to far-infrared images of the two dwarf compact elliptical galaxies that are satellites of M31, NGC 185, and NGC 147, obtained with the Spitzer Space Telescope. Spitzer's high sensitivity and spatial resolution enable us for the first time to look directly into the detailed spatial structure and properties of the dust in these systems. The images of NGC 185 at 8 and 24 μm display a mixed morphology characterized by a shell-like diffuse emission region surrounding a central concentration of more intense infrared emission. The lower resolution images at longer wavelengths show the same spatial distribution within the central 50 but beyond this radius, the 160 μm emission is more extended than that at 24 and 70 μm. On the other hand, the dwarf galaxy NGC 147, located only a small distance away from NGC 185, shows no significant infrared emission beyond 24 μm and therefore its diffuse infrared emission is mainly stellar in origin. For NGC 185, the derived dust mass based on the best fit to the spectral energy distribution is 1.9 × 10^3 M_⊙, implying a gas mass of 3.0 × 10^5 M_⊙. These values are in agreement with those previously estimated from infrared as well as CO and H I observations and are consistent with the predicted mass return from dying stars based on the last burst of star formation 1 × 10^9 yr ago. Based on the 70-160 μm flux density ratio, we estimate a temperature for the dust of ~17 K. For NGC 147, we obtain an upper limit for the dust mass of 4.5 × 10^2 M_⊙ at 160 μm (assuming a temperature of ~20 K), a value consistent with the previous upper limit derived using Infrared Space Observatory observations of this galaxy. In the case of NGC 185, we also present full 5-38 μm low-resolution (R ~ 100) spectra of the main emission regions. The Infrared Spectrograph spectra of NGC 185 show strong polycyclic aromatic hydrocarbons emission, deep silicate absorption features and H_2 pure rotational line ratios consistent with having the dust and molecular gas inside the dust cloud being impinged by the far-ultraviolet radiation field of a relatively young stellar population. Therefore, based on its infrared spectral properties, NGC 185 shows signatures of recent star formation (a few ×10^8 yr ago), although its current star formation rate is quite low.

A SPITZER HIGH-RESOLUTION MID-INFRARED SPECTRAL ATLAS OF STARBURST GALAXIES

We present an atlas of Spitzer/IRS high resolution (R~600) 10-37um spectra for 24 well known starburst galaxies. The spectra are dominated by fine-structure lines, molecular hydrogen lines, and emission bands of polycyclic aromatic hydrocarbons. Six out of the eight objects with a known AGN component show emission of the high excitation [NeV] line. This line is also seen in one other object (NGC4194) with, a priori, no known AGN component. In addition to strong polycyclic aromatic hydrocarbon emission features in this wavelength range (11.3, 12.7, 16.4um), the spectra reveal other weak hydrocarbon features at 10.6, 13.5, 14.2um, and a previously unreported emission feature at 10.75um. An unidentified absorption feature at 13.7um is detected in many of the starbursts. We use the fine-structure lines to derive the abundance of neon and sulfur for 14 objects where the HI 7-6 line is detected. We further use the molecular hydrogen lines to sample the properties of the warm molecular gas. Several basic diagrams characterizing the properties of the sample are also shown. We have combined the spectra of all the pure starburst objects to create a high S/N template, which is available to the community.

Polycyclic aromatic hydrocarbon selected galaxies

<i>Context. <i/>This is the fourth in a series of papers based on the ISOCAM Parallel Survey at 6.7 <i>μ<i/>m. While the first three papers have been devoted to active galactic nuclei (AGN), here we report on emission-line galaxies without AGN signatures in their optical spectra. <i>Aims. <i/> Polycyclic aromatic hydrocarbon (PAH) emission has been found in both starbursts and modestly starforming galaxies, but the relation between starforming activity and PAH luminosity is still a matter of debate. The different correlation degrees could be caused by the variety of optical and far-infrared sample selection criteria. In order to obtain a census of the typical properties of PAH emitting galaxies, we here study moderately distant galaxies which have been selected by their PAH emission.<i>Methods. <i/>Combining the ISOCAM Parallel Survey at 6.7 <i>μ<i/>m with 2MASS we have colour-selected a sample of 120 candidates for strong PAH emission. We obtained optical and mid-infrared spectra of 75 and 19 sources, respectively, and analysed IRAS-ADDSCANs and available Spitzer 3.6–160 <i>μ<i/>m photometry. <i>Results. <i/> The Spitzer mid-infrared spectra exhibit clear PAH features and corroborate that our photometric selection criteria trace the PAH emission of galaxies fairly well. The optical spectra show emission lines, at median redshift , as well as H and Ca II absorption, indicating ongoing starformation as well as post-starbursts. The mid- and far-infrared spectral energy distributions (SEDs) provide evidence for a broad range of far-infrared (FIR) luminosities (), but in general the dust is colder ( K, ) than for starburst galaxies like M 82 ( K). For most galaxies the monocromatic luminosity peaks at about equal height at optical, 6.7 <i>μ<i/>m (PAH) and FIR wavelengths. In about 15% of the sources the FIR luminosity exceeds the optical and PAH energy output by about a factor 5–10 despite the cool dust temperature; in these galaxies a large dust mass of 10<sup>8<sup/>–10<sup>9<sup/> is inferred. <i>Conclusions. <i/>At moderate distance (), PAH selected galaxies turn out to be a quite heterogeneous population of dust-rich, partly infrared-luminous galaxies but mostly cool with a range of post-starburst signatures and starforming activity which appears to be rather modest relative to the entire gas content (derived from the dust mass and assuming a standard gas/dust ratio). Our results on PAH selected galaxies question the often expressed interpretation that the majority of high redshift galaxies detected in 15 and 24 <i>μ<i/>m surveys are dominated by powerful ongoing starbursts with high starforming efficiency.

Wind-swept clouds and possible triggered star formation associated with the supernova remnant G357.7+0.3

We present evidence for interaction between the supernova remnant (SNR) G357.7+0.3 and nearby molecular clouds, leading to the formation of wind-swept structures and bright emission rims. These features are not observed at visual wavelengths, but are clearly visible in mid-infrared mapping undertaken using the Spitzer Space Telescope. Analysis of one of these clouds, the bright cometary structure G357.46+0.60, suggests that it contains strong polycyclic aromatic hydrocarbon emission features in the 5.8 and 8.0 μm photometric bands, and that these are highly variable over relatively small spatial scales. The source is also associated with strong variations in electron density; a far-infrared continuum peak associated with dust temperatures of ∼30 K; and has previously been observed in the 1720 MHz maser transition of OH, known to be associated with SNR shock excitation of interstellar clouds. This source also appears to contain a young stellar object (YSO) within the bright rim structure, with a steeply rising spectrum between 1.25 and 24 μm. If the formation of this star has been triggered recently by the SNR, then YSO modelling suggests a stellar mass ∼5–10 M⊙, and luminosity LYSO∼102–2 × 103 L⊙. Finally, it is noted that a further, conical emission region appears to be associated with the Mira V1139 Sco, and it is suggested that this may represent the case of a Mira outflow interacting with a SNR. If this is the case, however, then the distance to the SNR must be ∼half of that determined from CS J= 2–1 and 3–2 line radial velocities.

ISO spectroscopy of disks around Herbig Ae/Be stars

We have investigated the infrared spectra of all 46 Herbig Ae/Be stars for which spectroscopic data are available in the ISO data archive. Our quantitative analysis of these s pectra focuses on the emission bands at 3.3, 6.2, 7.7, 8.6 and 11.2 micron, linked to polycyclic aromatic hydrocarbons (PAHs), the nanodiamond-related features at 3.4 and 3.5 micron, the amorphous 10 micron silicate band and the crystalline silicate band at 11.3 micron. We have detected PAH emission in 57% of the Herbig stars in our sample. Although for most of these sources the PAH spectra are similar, there are clear examples of differences in the PAH spectra within our sample which can be explained by differences in PAH size, chemistry and/or ionization. Amorphous silicate emission was detected in the spectra of 52% of the sample stars, amorphous silicate absorption in 13%. We have detected crystalline silicate emission in 11 stars (24% of our sample), of which four (9%) also display strong PAH emission. We have classified the sample sources accordin g to the strength of their mid-IR energy distribution. The systems with stronger mid-infared (20-100� m) excesses relative to their near-infrared (1-5� m) excess display significantly more PAH emission than those with weaker mid-infrared excesses. There are no pronounced differences in the behaviour of the silicate feature between the two groups. This provides strong observational support for the disk models by Dullemond et al. (2001), in which systems with a flaring disk geometry display a strong mi d-infrared excess, whereas those with disks that are strongly shadowed by the puffed-up inner rim of the disk only display modest amounts of mid-infrared emission. Since the silicates are expected to be produced mainly in the warm inner disk regions, no large differences in silicate behaviour are expected between the two groups. In contrast to this, the PAH emission is expected to be produced mainly in the part of the disk atmosphere that is directly exposed to radiation from the central star. In th is model, self-shadowed disks should display weaker PAH emission than flared disks, consistent with our observations.

Spitzer Infrared Spectrograph Spectroscopy of the Prototypical Starburst Galaxy NGC 7714

We present observations of the starburst galaxy NGC 7714 with the Infrared Spectrograph on board the Spitzer Space Telescope. The spectra yield a wealth of ionic and molecular features that allow a detailed characterization of its properties. NGC 7714 has an H II region-like spectrum with strong polycyclic aromatic hydrocarbon emission features. We find no evidence for an obscured active galactic nucleus, and with [Ne ]/[Ne ] ≈ 0.73, NGC 7714 lies near the upper end of normal-metallicity starburst galaxies. With very little silicate absorption and a temperature of the hottest dust component of 340 K, NGC 7714 is the perfect template for a young, unobscured starburst.

First Mid‐Infrared Spectrum of a Faint High‐ z Galaxy: Observations of CFRS 14.1157 with the Infrared Spectrograph on the Spitzer Space Telescope

The unprecedented sensitivity of the Infrared Spectrograph on the Spitzer Space Telescope allows for the first time the measurement of mid-infrared spectra from 14 to 38 μm of faint high-z galaxies. This unique capability is demonstrated with observations of sources having 16 μm fluxes of 3.6 mJy (CFRS 14.1157) and 0.35 mJy (CFRS 14.9025). A spectral-fitting technique is illustrated that determines the redshift by fitting emission and absorption features characteristic of nearby galaxies to the spectrum of an unknown source. For CFRS 14.1157, the measured redshift is z = 1.00 ± 0.20, in agreement with the published result of z = 1.15. The spectrum is dominated by emission from an active galactic nucleus, similar to the nucleus of NGC 1068, rather than a typical starburst with strong polycyclic aromatic hydrocarbon emission like M82. Such spectra will be crucial in characterizing the nature of newly discovered distant galaxies, which are too faint for optical follow-up.

The two-points angular correlation function at large angular scale for Abell cluster of galaxies: AAnS 34/2 (1993)140–147

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 ∼5–8 μm emission continuum, strong emission bands from polycyclic aromatic hydrocarbon (PAH) molecules, and prominent 10 μm silicate emission. The steep ∼5–8 μm 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 ∼5–8 μm continuum to be consistent with that observed in IRAS F10398+1455 and IRAS F21013–0739. The flat ∼5–8 μm 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 ∼5–8 μm 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.