Fine-structure Line Emission Research Articles

DUST-BOUNDED ULTRALUMINOUS INFRARED GALAXIES: MODEL PREDICTIONS FOR INFRARED SPECTROSCOPIC SURVEYS

The observed faintness of infrared fine-structure line emission along with the warm far-infrared (FIR) colors of ultraluminous infrared galaxies (ULIRGs) is a long-standing problem. In this work, we calculate the line and continuum properties of a cloud exposed to an Active Galactic Nucleus (AGN) and starburst spectral energy distribution (SED). We use an integrated modeling approach, predicting the spectrum of ionized, atomic, and molecular environments in pressure equilibrium. We find that the effects of high ratios of impinging ionizing radiation density to particle density (i.e. high ionization parameters, or U) can reproduce many ULIRG observational characteristics. Physically, as U increases, the fraction of UV photons absorbed by dust increases, corresponding to fewer photons available to photoionize and heat the gas, producing what is known as a "dust-bounded" nebula. We show that high U effects can explain the "[C II] deficit", the ~1 dex drop in the [C II] 158 micron /FIR ratio seen in ULIRGs when compared to starburst or normal galaxies. Additionally, by increasing U through increasing the ionizing photon flux, warmer dust and thus higher IRAS F(60)/F(100) ratios result. High U effects also predict an increase in [O I]63 micron /[C II] 158 micron and a gradual decline in [O III] 88 micron /FIR, similar to the magnitude of the trends observed, and yield a reasonable fit to [Ne V]14 micron /FIR ratio AGN observations.

UNUSUAL DUST EMISSION FROM PLANETARY NEBULAE IN THE MAGELLANIC CLOUDS

We present a Spitzer Space Telescope spectroscopic study of a sample of 25 planetary nebulae in the Magellanic Clouds. The low-resolution modules are used to analyze the dust features present in the infrared spectra. This study complements a previous work by the same authors where the same sample was analyzed in terms of neon and sulfur abundances. Over half of the objects (14) show emission of polycyclic aromatic hydrocarbons, typical of carbon-rich dust environments. We compare the hydrocarbon emission in our objects to those of Galactic HII regions and planetary nebulae, and LMC/SMC HII regions. Amorphous silicates are seen in just two objects, enforcing the now well-known-fact that oxygen-rich dust is less common at low metallicities. Besides these common features, some planetary nebulae show very unusual dust. Nine objects show a strong silicon carbide feature at 11um and twelve of them show magnesium sulfide emission starting at 25um. The high percentage of spectra with silicon carbide in the Magellanic Clouds is not common. Two objects show a broad band which may be attributed to hydrogenated amorphous carbon and weak low-excitation atomic lines. It is likely that these nebulae are very young. The spectra of the remaining eight nebulae are dominated by the emission of fine-structure lines with a weak continuum due to thermal emission of dust, although in a few cases the S/N in the spectra is low, and weak dust features may not have been detected.

A Compendium of Far‐Infrared Line and Continuum Emission for 227 Galaxies Observed by theInfrared Space Observatory

Far-infrared line and continuum fluxes are presented for a sample of 227 galaxies observed with the Long Wavelength Spectrometer on the Infrared Space Observatory. The galaxy sample includes normal star-forming systems, starbursts, and active galactic nuclei covering a wide range of colors and morphologies. The data set spans some 1300 line fluxes, 600 line upper limits, and 800 continuum fluxes. Several fine-structure emission lines are detected that arise in either photodissociation or H II regions: [O III] 52 μm, [N III] 57 μm, [O I] 63 μm, [O III] 88 μm, [N II] 122 μm, [O I] 145 μm, and [C II] 158 μm. Molecular lines such as OH at 53, 79, 84, 119, and 163 μm, and H2O at 58, 66, 75, 101, and 108 μm are also detected in some galaxies. In addition to those lines emitted by the target galaxies, serendipitous detections of Milky Way [C II] 158 μm and an unidentified line near 74 μm in NGC 1068 are also reported. Finally, continuum fluxes at 52, 57, 63, 88, 122, 145, 158, and 170 μm are derived for a subset of galaxies in which the far-infrared emission is contained within the ~75 ISO LWS beam. The statistics of this large database of continuum and line fluxes, including trends in line ratios with the far-infrared color and infrared-to-optical ratio, are explored.

INFRARED STUDY OF THE SOUTHERN GALACTIC STAR-FORMING REGIONS ASSOCIATED WITH IRAS 10049-5657 AND IRAS 10031-5632

The physical conditions of the interstellar medium and stellar components in the regions of the southern Galactic star forming complexes associated with IRAS 10049-5657 and IRAS 10031-5632 have been investigated. These regions have been mapped simultaneously in two far infrared bands lambda_eff ~ 150 & 210 micron, with ~ 1' angular resolution using the TIFR 1-m balloon borne telescope. Spatial distribution of the temperature of cool dust and optical depth at 200 micron have been obtained. Using the 2MASS sources, the stellar populations of the embedded young clusters have been studied. A rich cluster of OB stars in the IRAS 10049-5657 region has been found with a cluster radius ~ 2 pc. The source in the cluster closest to the IRAS peak, lies above the ZAMS curve of spectral type O5 in the colour-magnitude diagram. Unlike IRAS 10049-5657, a small cluster comprising of a few deeply embedded sources is seen at the location of IRAS 10031-5632. Self consistent radiative transfer modelling aimed at extracting important physical and geometrical details of the two IRAS sources show that the best fit models are in good agreement with the observed spectral energy distributions. The geometric details of the associated cloud and optical depths (at 100 micron) have been estimated. A uniform density distribution of dust and gas is implied for both the sources. In addition, the infrared ionic fine-structure line emission from gas has been modelled for both these regions and compared with data from IRAS-Low Resolution Spectrometer. For IRAS 10049-5657, the observed and modelled luminosities for most lines agree to within a factor of four while for IRAS 10031-5632, we find a discrepancy of a factor of 100.

Mid-Infrared Characteristics of Star-Forming Complexes in Irregular Galaxies

ABSTRACTWe present Spitzer Space Telescope Infrared Spectrograph 10Y37 m high-resolution spectral maps of 11 star-formingregionsinfiveirregulargalaxies.Wemeasureorplaceupperlimitsonnebularfine-structurelines,molecularlines from warm H 2 , and various polycyclic aromatic hydrocarbon (PAH) features in order to probe the hardness ofthestellar radiation field,the electrondensity ofthe nebulae, andthephysicalenvironment in whichPAHsare beingheated.Thenebulaehavesimilarexcitations,correspondingtoamidtypeOstar,andelectrondensitiesarelowinmostregions. PAHs were detected in all but two nebulae, and PAH emission relative to stars or H emission tends to behigheringalaxieswithhigheroxygenabundances.Thepopulationof PAHsappearstobedifferentinImgalaxiesthanit is in spirals. We model emission from warm H 2 in one nebula to obtain a temperature of 800 þ440 340 K, a neutral gasdensity 310 5 cm ,andafar-UVradiationfieldof G 0 10 3 .Variationsinexcitationacrossthenebulaearesmall,butthePAH11.2 memissionvariesbyfactorsof 3Y4acrossthemaps.DifferencesinthelocalHiorHiienvironmentofstar-forming regions result in only mild differences in mid-IR ratios of nebular fine-structure line emission amongIm galaxies. Variations in PAH emission, however, are more evident.Key words: galaxies: individual (NGC 1156, Haro 14, Holmberg II, NGC 1705, NGC 6822) —galaxies: irregular — galaxies: ISM — galaxies: kinematics and dynamics — stars: formation1. INTRODUCTIONNearbydwarf irregular(dIm)galaxiesprovideauniqueoppor-tunitytostudytheinterstellarmedium(ISM)of low-metallicitysystemsforuseininterpretingobservationsof high-redshiftgal-axies. This is especially urgent now, as we are beginning todirectly probe star formation in young galaxies (see review bySolomonVLisenfeldFBolattoetal.2000;Madden2000; Hunter et al. 2001; Lisenfeld et al. 2002; Hoopes et al.2004;Kiuchietal.2004).Inaddition,dImmolecularcloudsare,to a greater extent than in spirals, the dense cores of large Hiclouds. This can be understood as a result of the low metallic-ity of these galaxies and the higher density necessary to shieldthemoleculesfromtheambientstellarradiationfield(Federmanet al. 1979; Franco & Cox 1986; Maloney & Black 1988;Elmegreen1989;Rubioetal.1991;Bolattoetal.1999;BlitzR Leroy et al. 2005, 2006; Blitz et al. 2007).The IR spectral region provides detailed diagnostics of theISMofgalaxies,andtheincreasedsensitivityof theSpitzerSpaceTelescopeenablesusforthefirsttimetoextendIRstudiestotyp-ical dIm galaxies (rather than solely starbursts). For this reason,we have analyzed Spitzer Infrared Spectrograph (IRS; Houcket al. 2004a) high-resolution spectroscopy at 10Y37 mof11 star-forming regions in three Im galaxies (Holmberg II,NGC1156,andNGC6822)andtwobluecompactdwarf (BCD)galaxies(Haro14andNGC1705).Thespectraldataof NGC1156and Haro 14 were obtained as part of our own GO program, andthe other data are from theSpitzer Infrared Nearby GalaxiesSurvey (SINGS; Kennicutt et al. 2003) Legacy database.NGC 1156, NGC 6822, and Holmberg II have star-formingregionsscatteredovertheirdisks.Sevenstar-formingregionswereobservedinNGC6822andseveralinHolmbergII.InNGC1156the highest column density gas complex was targeted, and itcontainsonlymodestH emission(Hunter&Elmegreen2004).BycontrasttotheImgalaxies,theHiinHaro14isconcentratedat the center of the galaxy, forming an enormous gas and star-formingcomplex.AndNGC1705isfamousforhavingrecentlyformed a superstar cluster.Thus, with thesegalaxieswe can ex-aminethepropertiesof gasandstar-formingcomplexesindiffer-ent local environments.We use these data to examine the interaction of massive starswith the ISM in star-forming regions. We measure or place up-per limits on nebular fine-structure lines, molecular lines fromwarm H

Neon Fine‐Structure Line Emission by X‐Ray Irradiated Protoplanetary Disks

Using a thermal-chemical model for the generic T Tauri disk of D'Alessio and colleagues, we estimate the strength of the fine-structure emission lines of Ne II and Ne III at 12.81 and 15.55 μm that arise from the warm atmosphere of the disk exposed to hard stellar X-rays. The Ne ions are produced by the absorption of keV X-rays from the K shell of neutral Ne, followed by the Auger ejection of several additional electrons. The recombination of the Ne ions is slow because of weak charge transfer with atomic hydrogen in the case of Ne+2 and by essentially no charge transfer for Ne+. For a distance of 140 pc, the 12.81 μm line of Ne II has a flux ~10-14 ergs cm-2 s-1, which should be observable with the Spitzer Infrared Spectrometer and suitable ground-based instrumentation. The detection of these fine-structure lines would clearly demonstrate the effects of X-rays on the physical and chemical properties of the disks of young stellar objects and provide a diagnostic of the warm gas in protoplanetary disk atmospheres. They would complement the observed H2 and CO emission by probing vertical heights above the molecular transition layer and larger radial distances that include the location of terrestrial and giant planets.

The interaction of young massive stars with their environment

Using the 15-m Swedish ESO Sub-millimeter Telescope (SEST), CO, HCN, and HCO+ observations of the galactic star-forming region NGC 6334 FIR II are presented, complemented by [C i] 3PP0 and 3PP1 data from the Atacama Pathfinder Experiment (APEX 12-m telescope). Embedded in the extended molecular cloud and associated with the H ii region NGC 6334–D, there is a molecular “void”. [C i] correlates well with 13CO and other molecular lines and shows no rim brightening relative to molecular cloud regions farther off the void. While an interpretation in terms of a highly clumped cloud morphology is possible, with photon dominated regions (PDRs) reaching deep into the cloud, the data do not provide any direct evidence for a close association of [C i] with PDRs. Kinetic temperatures are ~40-50 K in the molecular cloud and 200 K toward the void. CO and [C i] excitation temperatures are similar. A comparison of molecular and atomic fine structure line emission with the far infrared and radio continuum as well as the distribution of 2.2m H2 emission indicates that the well-evolved H ii region expands into a medium that is homogeneous on pc-scales. If the H2 emission is predominantly shock excited, both the expanding ionization front (classified as subsonic, “D-type”) and the associated shock front farther out (traced by H2) can be identified, observationally confirming for the first time a classical scenario that is predicted by evolutionary models of Hii regions. Integrated line intensity ratios of the observed molecules are determined, implying a mean C18O/C17O abundance ratio of 4.13 ± 0.13 that reflects the 18O/17O isotope ratio. This ratio is consistent with values determined in nearby clouds. Right at the edge of the void, however, the oxygen isotope ratio might be smaller.

Mid-Infrared Diagnostics of LINER s

We report results from the first mid-infrared spectroscopic study of a comprehensive sample of 33 LINERs, observed with the Spitzer Space Telescope. We compare the properties of two different LINER populations: infrared-faint LINERs, with LINER emission arising mostly in compact nuclear regions, and infrared-luminous LINERs, which often show spatially extended (non-AGN) LINER emission. We show that these two populations can be easily distinguished by their mid-infrared spectra in three different ways: (i) their mid-IR spectral energy distributions (SEDs), (ii) the emission features of polycyclic aromatic hydrocarbons (PAHs), and (iii) various combinations of IR fine-structure line ratios. IR-luminous LINERs show mid-IR SEDs typical of starburst galaxies, while the mid-IR SEDs of IR-faint LINERs are much bluer. PAH flux ratios are significantly different in the two groups. Fine structure emission lines from highly excited gas, such as [O IV], are detected in both populations, suggesting the presence of an additional AGN also in a large fraction of IR-bright LINERs, which contributes little to the combined mid-IR light. The two LINER groups occupy different regions of mid-infrared emission-line excitation diagrams. The positions of the various LINER types in our diagnostic diagrams provide important clues regarding the power source of each LINER type. Most of these mid-infrared diagnostics can be applied at low spectral resolution, making AGN- and starburst-excited LINERs distinguishable also at high redshifts.

SpitzerQuasar and ULIRG Evolution Study (QUEST). I. The Origin of the Far‐Infrared Continuum of QSOs

This paper addresses the origin of the far-infrared (FIR) continuum of QSOs, based on the Quasar and ULIRG Evolution Study (QUEST) of nearby QSOs and ULIRGs using observations with the Spitzer Space Telescope. For 27 Palomar-Green QSOs at z ≲ 0.3, we derive luminosities of diagnostic lines ([Ne II] 12.8 μm, [Ne V] 14.3 μm, [O IV] 25.9 μm) and emission features (PAH 7.7 μm emission, which is related to star formation), as well as continuum luminosities over a range of mid- to far-infrared wavelengths between 6 and 60 μm. We detect star-formation-related PAH emission in 11/26 QSOs and fine-structure line emission in all of them, often in multiple lines. The detection of PAHs in the average spectrum of sources that lack individual PAH detections provides further evidence for the widespread presence of PAHs in QSOs. Similar PAH/FIR and [Ne II]/FIR ratios are found in QSOs and in starburst-dominated ULIRGs and lower luminosity starbursts. We conclude that the typical QSO in our sample has at least 30% but likely most of the far-infrared luminosity (~10^(10)-10^(12) L_☉) arising from star formation, with a tendency for larger star formation contribution at the largest FIR luminosities. In the QSO sample, we find correlations between most of the quantities studied including combinations of AGN tracers and starburst tracers. The common scaling of AGN and starburst luminosities (and fluxes) is evidence for a starburst-AGN connection in luminous AGN. Strong correlations of far-infrared continuum and starburst related quantities (PAH, low-excitation [Ne II]) offer additional support for the starburst origin of far-infrared emission.

Siii], [Feii], [Cii], and H2Emission from Massive Star‐forming Regions

We calculate the [Si II] 34.8 μm, [Fe II] 26.0 μm, and [C II] 158 μm infrared fine-structure emission that may arise from ionized gas (H II regions) and/or associated neutral gas (photodissociation regions [PDRs]) in massive star-forming regions. Assuming thermal pressure balance between an H II region and a PDR, the relative amounts of fine-structure line emission from the H II region and PDR depend on the electron density and resulting thermal pressure in the H II region, gas-phase abundances of the emitting species, and the UV spectrum from the stellar population producing the H II region. For normal metallicity, we find that [C II] emission is always dominated by PDRs, while [Si II] and [Fe II] are dominated by PDRs for H II regions with electron density ne 10 cm-3. We also calculate the H2 0-0 S(0), 0-0 S(1), 0-0 S(2), and 0-0 S(3) pure rotational line emission arising from the PDR at the atomic-to-molecular interface. The overall intensity of the H2 line emission directly traces warm molecular mass, while H2 line ratios constrain the PDR temperature, gas density, and far-ultraviolet field strength. Models of the integrated emission of [Si II], [Fe II], [C II], and H2 from Galactic and extragalactic star-forming regions are presented for use in interpreting observations with Spitzer, ISO, SOFIA, and the Herschel Space Observatory. We compare our results with observations of the Galactic source NGC 2023 (an individual H II/photodissociation region in Orion), the inner regions of the Milky Way, and the central regions of the nearby star-forming spiral galaxy NGC 7331. We also compare our results with recently published similar work by Abel and coworkers.

Critical Metallicity and Fine‐Structure Emission of Primordial Gas Enriched by the First Stars

The influence of the first stars on the formation of second-generation objects at high redshift may be determined, in part, by their metal enrichment of surrounding gas. At a critical metallicity, Zcrit, primordial gas cools more efficiently by fine-structure lines of CII (157.74 um), OI (63.18 um, 145.5 um), SiII (34.8 um), and FeII (25.99 um, 35.35 um) than by HI or H2 emission. This cooling may alter the process of fragmentation into smaller units. We study the time-dependent cooling of primordial gas enriched by heavy elements from early massive stars, particularly O, Si, and Fe. We define Zcrit as the point when the total cooling rate by metals plus H2 equals the adiabatic compressional heating. We explore two metallicity scenarios: (1) a single metallicity for all heavy elements; (2) individual metallicities (Z_C, Z_O, Z_Si, Z_Fe) from theoretical supernova yields. For dense gas [n > 10^3 cm^(-3)] with metals in relative solar abundances, fragmentation occurs at Zcrit ~ 10^(-3.5) Z_sun. However, for lower density gas, [n = 1-100 cm^(-3)], particularly in halos enriched in Si, O, and Fe, we find Zcrit = 0.1-1% Zsun. The critical metallicity approaches a minimum value at high-n set by efficient LTE cooling, with thermalized level populations of fine-structure states and H2 rotational states (J = 2 and J = 3). Primordial clouds of 10^8 Msun at 200K are detectable in redshifted fine-structure lines, with far-infrared fluxes between 10^-22 and 10^-21 W/m^2. For metallicities Z_O = 10^(-3) and molecular fractions f(H2) = 10^(-3) the fine-structure emission lines of OI, SiII, and FeII could be 10^2 - 10^3 times stronger than the H2 rotational lines at 28.22 um (J = 2-0) and 17.03 um (J = 3-1).

The Far‐Infrared Emission Line and Continuum Spectrum of the Seyfert Galaxy NGC 1068

We report on the analysis of the first complete far-infrared spectrum (43-197 microns) of the Seyfert 2 galaxy NGC 1068 as observed with the Long Wavelength Spectrometer (LWS) onboard the Infrared Space Observatory (ISO). In addition to the 7 expected ionic fine structure emission lines, the OH rotational lines at 79, 119 and 163 microns were all detected in emission, which is unique among galaxies with full LWS spectra, where the 119 micron line, where detected, is always in absorption. The observed line intensities were modelled together with IS0 Short Wavelength Spectrometer (SWS) and optical and ultraviolet line intensities from the literature, considering two independent emission components: the AGN component and the starburst component in the circumnuclear ring of approximately 3kpc in size. Using the UV to mid-IR emission line spectrum to constrain the nuclear ionizing continuum, we have confirmed previous results: a canonical power-law ionizing spectrum is a poorer fit than one with a deep absorption trough, while the presence of a big blue bump is ruled out. Based on the instantaneous starburst age of 5 Myr constrained by the Br gamma equivalent width in the starburst ring, and starburst synthesis models of the mid- and far-infrared fine-structure line emission, a low ionization parameter (U=10(exp -3.5)) and low densities (n=100 cm (exp -3)) are derived. Combining the AGN and starburst components, we succeed in modeling the overall UV to far-IR atomic spectrum of SGC 1068, reproducing the line fluxes to within a factor 2.0 on average with a standard deviation of 1.4. The OH 119 micron emission indicates that the line is collisionally excited, and arises in a warm and dense region. The OH emission has been modeled using spherically symmetric, non-local, non-LTE radiative transfer models. The models indicate that the bulk of the emission arises from the nuclear region, although some extended contribution from the starburst is not ruled out. The OH abundance in the nuclear region is expected to be approximately 10(exp -5), characteristic of X-ray dominated regions.

ISO observations of the Galactic center interstellar medium

The 500 central pc of the Galaxy (hereafter GC) exhibit a widespread gas component with a kinetic temperature of 100–200 K. The bulk of this gas is not associated to the well-known thermal radio continuum or far infrared sources like Sgr A or Sgr B. How this gas is heated has been a longstanding problem. With the aim of studying the thermal balance of the neutral gas and dust in the GC, we have observed 18 molecular clouds located at projected distances far from thermal continuum sources with the Infrared Space Observatory (ISO). In this paper we present observations of several fine structure lines ([Oi] 63 and 146 μm, [Cii] 158 μm, [Si ii] 35 μm, [S i] 25 μm and [Fe ii] 26 μm), which are the main coolants of the gas with kinetic temperatures of several hundred K. We also present the full continuum spectra of the dust between 40 and 190 μm. All the clouds exhibit a cold dust component with a temperature of ~15 K. A warmer dust component is also required to fit the spectra. The temperature of this dust component changes between 27 and 42 K from source to source. We have compared the gas and the dust emission with the predictions from J-type and C-type shocks and photodissociation region (PDRs) models. We conclude that the dust and the fine structure lines observations are best explained by a PDR with a density of 103 cm-3 and an incident far-ultraviolet field 103 times higher than the local interstellar radiation field. The fine structure line emission arises in PDRs in the interface between a diffuse ionized gas component and the dense molecular clouds. The [Cii] 158 μm and [Si ii] 35 μm lines also have an important contribution from the ionized gas component. PDRs can naturally explain the discrepancy between the gas and the dust temperatures. However, these PDRs can only account for 10–30% of the total H2 column density with a temperature of ~150 K. We discuss other possible heating mechanisms for the rest the warm molecular gas, such as non-stationary PDRs, X-ray Dominated Regions (XDRs) or the dissipation of supersonic turbulence.

A joint mid-infrared spectroscopic and X-ray imaging investigation of LINER galaxies

We present a comprehensive comparative high resolution mid-IR spectroscopic and X-ray imaging investigation of LINERs using archival observations from the ISO-SWS and the Chandra Advanced CCD Imaging Spectrometer. Although the sample is heterogenous and incomplete, this is the first comprehensive study of the mid-infrared fine structure line emission of LINERs. These results have been compared with similar observations of starburst galaxies and AGN. We find that LINERs very clearly fall between starbursts and AGN in their mid-IR fine structure line spectra, showing /LFIR and / ratios, both measures of the dominant nuclear energy source in dust-enshrouded galaxies, intermediate between those of AGN and starbursts. Chandra imaging observations of the LINERs reveal hard nuclear point sources morphologically consistent with AGN in most (67%) of the sample, with a clear trend with IR-brightness. Most LINERs that show a single dominant hard compact X-ray core are IR-faint (LFIR/), whereas most LINERs that show scattered X-ray sources are IR-bright. A comparative X-ray/mid-IR spectroscopic investigation of LINERs reveals some puzzling results. Objects that display strong hard nuclear X-ray cores should also display high excitation lines in the IR. However, we find two LINERs disagree with this expectation. The galaxy NGC 404 shows weak soft X-ray emission consistent with a starburst but has the most prominent highest excitation mid-IR spectrum of our entire sample. Using IR emission line diagnostics alone, this galaxy would be classified as hosting a dominant AGN. Conversely, the IR luminous LINER NGC 6240 has an extremely luminous binary AGN as revealed by the X-rays but shows weak IR emission lines. With the advent of SIRTF, and future IR missions such as Herschel and JWST, it is increasingly critical to determine the origin of these multiwavelength anomalies.

A Uniform Database of 2.2–16.5 Micron Spectra from the ISOCAM CVF Spectrometer

We present all ISOCAM circular variable filter (CVF) spectra that cover more than one-third of the 2.2-16.5 μm spectral range of the instrument. The 364 spectra have been classified according to the classification system of Kraemer et al., as modified by Hodge et al. to account for the shorter wavelength range. Prior to classification, the spectra were processed and recalibrated to create a uniform database. Aperture photometry was performed at each wavelength centered on the brightest position in each image field and the various spectral segments merged into a single spectrum. The aperture was the same for all scale sizes of the images. Since this procedure differs fundamentally from that used in the initial ISOCAM calibration, a recalibration of the spectral response of the instrument was required for the aperture photometry. The recalibrated spectra and the software used to create them are available to the community on-line via the ISO Data Archive. Several new groups were added to the KSPW system to describe spectra with no counterparts in either the SWS or PHT-S databases: CA, E/SA, UE/SA, and SSA. The zodiacal dust cloud provides the most common background continuum to the spectral features, visible in almost 40% of the processed sources. The most characteristic and ubiquitous spectral features observed in the CVF spectral atlas are those of the unidentified infrared bands (UIR), which are typically attributed to ultraviolet-excited fluorescence of large molecules containing aromatic hydrocarbons. The UIR features commonly occur superimposed on the zodiacal background (18%) but can also appear in conjunction with other spectral features, such as fine-structure emission lines or silicate absorption. In at least 13 of the galaxies observed, the pattern of UIR emission features has been noticeably shifted to longer wavelengths.

The Far‐Infrared Spectrum of the Sagittarius B2 Region: Extended Molecular Absorption, Photodissociation, and Photoionization

We present large-scale 9' × 27' (~25 pc × 70 pc) far-IR observations around Sgr B2 using the Long-Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO). The spectra are dominated by the strong continuum emission of dust, the widespread molecular absorption of light hydrides (OH, CH, and H2O), and the fine-structure lines of [N II], [N III], [O III], [C II], and [O I]. The widespread dust emission is reproduced by a cold component (Td ≃ 13-22 K) together with a warm component (Td ≃ 24-38 K), representing ≲10% of the dust opacity. The fine-structure line emission reveals a very extended component of ionized gas. The [O III] 52 μm/88 μm and [N III] 57 μm/[N II] 122 μm line intensity ratios show that the ionized gas has an average electron density of ~240 cm-3. The ionizing radiation can be characterized by a hard but diluted continuum, with effective temperatures of ~36,000 K and a Lyman continuum photon flux of ~1050.4 s-1. The spatial distribution of the ionizing sources with respect to the extended cloud and the clumpiness of the medium determine the large-scale effects of the radiation. Photodissociation regions (PDRs) can be numerous at the interface of the ionized and neutral gas. The analysis of the [C II] 158 μm and [O I] 63 and 145 μm lines indicates a far-UV radiation field of G0 ≃ 103-104 and a density of nH = 103-104 cm-3 in these PDRs. The widespread OH lines are reproduced by nonlocal radiative transfer models for clouds of moderate volume density (n ≃ 103-104 cm-3) at Tk ≳ 40-100 K. PDR models can explain the enhanced column density of species such as H2O, OH, and O0. However, they fail to reproduce the observed NH3/NH2/NH ≃ 100/10/1 abundance ratios. For N-bearing species, it seems that shock chemistry has to be invoked. The molecular richness in the outer layers of Sgr B2 is probed by the ISO/LWS Fabry-Pérot (~35 km s-1) detections toward Sgr B2(M), where more than 70 lines from 15 molecular and atomic species are observed at high signal-to-noise ratios.

The Trumpler 14 photodissociation region in the Carina Nebula

We report the results of observations of the fine-structure emission lines (C ) 158 µ ma nd (O )6 3µm using FIFI on the Kuiper Airborne Observatory (KAO) and the Long Wavelength Spectrometer (LWS) on board ISO, towards the molecular cloud associated with the stellar cluster Trumpler 14 (Tr 14) in the Carina Nebula. These data are compared with selected CO and CS transitions obtained with the SEST as well as IRAS and MSX images to produce a detailed view of the morphology and the physical conditions prevailing in the photodissociation region (PDR) at the interface between the ionized gas and the molecular dust lane. The relative intensity distribution observed for the various tracers is consistent with the stratification expected for a molecular cloud seen edge-on and exposed to a radiation field of ≈10 4 G0, which is dominated by the most massive stars of Tr 14. The grain photoelectric heating efficiency, � , is estimated to be ≈5 × 10 −3 and is comparable to other galactic PDRs. The molecular gas has a complicated velocity structure with a high velocity dispersion resulting from the impact of the stellar winds arising from Tr 14. There is evidence of small-scale clumping with a very low volume filling factor. Despite the rich concentration of massive O stars in Tr 14 we find that the parameters of the PDR are much less-extreme than those of the Orion and M 17 massive star-forming regions.

ISO spectroscopy of star formation and active nuclei in the luminous infrared galaxy NGC 6240

We present Infrared Space Observatory mid- and far-infrared spectroscopy of the merging galaxy NGC 6240, an object presenting many aspects of importance for the role of star formation and AGN activity in [ultra]luminous infrared galaxies. The mid-infrared spectrum shows starburst indicators in the form of low excitation fine-structure line emission and aromatic `PAH' features. A strong high excitation [OIV] line is observed which most likely originates in the Narrow Line Region of an optically obscured AGN. NGC 6240 shows extremely powerful emission in the pure rotational lines of molecular hydrogen. We argue that this emission is mainly due to shocks in its turbulent central gas component and its starburst superwind. The total shock cooling in infrared emission lines accounts for ~0.6% of the bolometric luminosity, mainly through rotational H_2 emission and the [OI] 63micron line. We analyse several ways of estimating the luminosities of the starburst and the AGN in NGC 6240 and suggest that the contributions to its bolometric luminosity are most likely in the range 50-75% starburst and 25-50% AGN.

Mapping of large scale 158 [CII] line emission: Orion A

We present the first results of an observational programme undertaken to map the fine structure line emission of singly ionized carbon ([ CII] 157 : 7409 mum) over extended regions using a Fabry Perot spectrometer newly installed at the focal plane of a 100 cm balloon- borne far- infrared telescope. This new combination of instruments has a velocity resolution of similar to 200 km s(-1) and an angular resolution of 1.'5. During the first flight, an area of 30' x 15' in Orion A was mapped. These observations extend over a larger area than previous observations, the map is fully sampled and the spectral scanning method used enables reliable estimation of the continuum emission at frequencies adjacent to the [ CII] line. The total [ CII] line luminosity, calculated by considering up to 20% of the maximum line intensity is 0.04% of the luminosity of the far- infrared continuum. We have compared the [ CII] intensity distribution with the velocity- integrated intensity distributions of (CO)-C-13(1- 0), CI(1- 0) and CO( 3- 2) from the literature. Comparison of the [ CII], [ CI] and the radio continuum intensity distributions indicates that the largescale [ CII] emission originates mainly from the neutral gas, except at the position of M 43, where no [ CI] emission corresponding to the [ CII] emission is seen. Substantial part of the [ CII] emission from here originates from the ionized gas. The observed line intensities and ratios have been analyzed using the PDR models by Kaufman et al. ( 1999) to derive the incident UV flux and volume density at a few selected positions. The models reproduce the observations reasonably well at most positions excepting the [ CII] peak ( which coincides with the position of theta(1) Ori C). Possible reason for the failure could be the simplifying assumption of a homogeneous plane parallel slab in place of a more complicated geometry.

Observations of SiO towards photon dominated regions

We report observations with the IRAM 30-m telescope of SiO(2-1), (3-2), and (5-4) towards the Orion Bar, S 140, NCC 2023 and NGC 7023. We detect SiO towards the Orion Bar and the S 140 photon dominated region (PDR) with an abundance of order 10-11 relative to . Towards the Orion Bar, SiO is extended and is not confined to a layer close to the ionization front. We put limits of 2- on [SiO]/[ ] towards several positions in NGC 2023 and NGC 7023. In order to interpret these data, we present model calculations of the SiO abundance in PDRs as a function of the incident radiation field and consider also the expected Si+ 35 fine structure line intensity. Our data suggest that silicon is very highly depleted onto dust grains within the molecular cloud but that a fraction of the order of ten percent of the silicon comes back into the gas phase in a layer close to the ionization front. We attempted to estimate whether the observed silicon fine structure line emission could originate in fully ionized gas and conclude that in the case of the Orion Bar, it is quite likely. Finally, a by-product of our observations was the detection of the 86773.43 MHz component of the line of HCO in all four sources confirming that this species is relatively abundant in PDRs.