Infrared Space Observatory Long-Wavelength Spectrometer Research Articles

The ISO LWS high-resolution spectral survey towards Sagittarius B2

A full spectral survey was carried out towards the giant molecular cloud complex, Sagittarius B2 (Sgr B2), using the Infrared Space Observatory (ISO) Long Wavelength Spectrometer (LWS) Fabry–Pérot mode. This provided complete wavelength coverage in the range 47–196 μm (6.38–1.53 THz) with a spectral resolution of 30–40 km s−1. This is a unique data set covering wavelengths inaccessible from the ground. It is an extremely important region of the spectrum as it contains both the peak of the thermal emission from dust, and crucial spectral lines of key atomic (O i, C ii, O iii, N ii and N iii) and molecular species (NH3, NH2, NH, H2O, OH, H3O+, CH, CH2, C3, HF and H2D+). In total, 95 spectral lines have been identified and 11 features with absorption depth greater than 3σ remain unassigned. Most of the molecular lines are seen in absorption against the strong continuum, whereas the atomic and ionic lines appear in emission (except for absorption in the O i 63 μm and C ii 158 μm lines). Sgr B2 is located close to the Galactic Centre and so many of the features also show a broad absorption profile due to material located along the line of sight. A full description of the survey data set is given with an overview of each detected species and final line lists for both assigned and unassigned features.

ITAL]ISO[/ITAL] LWS Observations of the Two Nearby Spiral Galaxies NGC 6946 and NGC 1313

We present the analysis of Infrared Space Observatory long-wavelength spectrometer (LWS) observations of the two nearby late-type galaxies NGC 1313 and NGC 6946. Both galaxies have been fully mapped in the [C II] far-infrared fine-structure line at 158 μm, and some regions have been observed also in the [O I] 63 μm and [N II] 122 μm lines. We use these observations to derive the physical properties of the atomic interstellar medium, to establish how they relate to other interstellar medium components (gas and dust), and to establish how they vary with different galaxy components such as nucleus, spiral arms, and disk. The [C II] line is the main cooling line of the atomic medium. In NGC 6946 and NGC 1313, its emission represents 0.8% of the infrared emission. Moreover, the [C II] emission can be spatially associated with three components: the nucleus, the star-forming regions in spiral arms, and the diffuse galaxy disk. This last component contributes 40% in NGC 6946 and ~30% in NGC 1313 to the total emission. We apply the photodissociation region (PDR) model by Kaufman et al. to derive PDR physical parameters responsible for the neutral atomic gas emission (G0, n, and Ts). The results do not significantly differ from what Malhotra et al. recently found by modeling the integrated emission of a sample of 60 normal galaxies. This suggests that the emission in each region under the LWS beam in NGC 6946 and NGC 1313 (corresponding to a linear size of ~1.5 kpc) is likely to arise from a mixture of components similar to the mixture producing the integrated emission of normal galaxies. However, some regions in NGC 6946 have a G0/n ratio ~2–3 times smaller than the mean value found for the normal galaxy sample (1.3), suggesting that the beam-averaged contribution of a less active component in these regions is higher than its contribution in the integrated emission of normal galaxies or, conversely, that the bulk of the integrated emission of the normal galaxies is dominated by a few active regions probably located in their nuclei. CO(1–0) and [C II] in NGC 6946 are well correlated, and the mean [C II]/CO ratio agrees with the mean integrated ratios of the normal galaxies sample. This value (~500) is a factor of ~2 less than the mean ratio found for a sample of normal galaxies observed with the Kupier Airborne Observatory (KAO) by Stacey et al. This difference is probably due to the fact that the KAO beam (55'') is smaller than the LWS beam (75''), such that the Stacey et al. KAO observations are likely to be more biased toward the nucleus of the galaxies and therefore toward more active regions. In NGC 1313 only four LWS regions have been observed in CO(1–0), and three of them were detected. The [C II]/CO(1–0) seems to systematically increase from the northeast to the south, along the S-shaped spiral arm, indicating that the interstellar medium conditions in NGC 1313 are much more inhomogeneous than the conditions in NGC 6946. H I and [C II] in NGC 6946 are completely uncorrelated, probably because they arise from different gas components: [C II] arises principally in dense and warm PDR and H I from diffuse (n 3 × 103 cm-3) gas. On the other hand, in NGC 1313 we successfully detect two distinct gas components: a cirrus-like component, for which H I and [C II] are weakly correlated as observed in our Galaxy, and a component associated with dense PDRs completely uncorrelated from H I as observed in NGC 6946. Finally, we find that the H I residing in dense gas surrounding the star-forming regions and presumably recently photodissociated constitutes a few percent of the total H I. In turn, this dense gas component produces most of the [C II] emission emitted by the atomic neutral medium, even if its contribution is lower in NGC 1313 than in NGC 6946. On the other hand, the [C II] emission arising from ionized gas is higher in NGC 1313 than in NGC 6946.

Far-Infrared Detection of H[TINF]3[/TINF]O[TSUP]+[/TSUP] in Sagittarius B2

We report the first far-infrared detection of the hydronium ion (H3O+) toward the Sagittarius B2 molecular cloud, near the Galactic center. Using the Infrared Space Observatory Long-Wavelength Spectrometer, we have detected three lines arising from the ν2 ground-state inversion mode (0+ → 0-) at 55.3 cm-1. All transitions are observed in absorption against the optically thick infrared continuum emission of the dust. Two different absorption regions can be distinguished, the H3O+ associated with the Sgr B2 extended envelope and that of foreground gas along the line of sight. The derived abundances for both components are rather similar, 1 × 10-9, and may explain the high water abundance found in the low-density gas toward Sgr B2 as its dissociative recombination leads to the formation of important molecules OH and H2O. Contribution of the Q(1, 1) line of para-H3O+ to the 212-101 line of H218O is also analyzed. This allows a better determination of water abundance from far-infrared observations. The corrected H2O abundances are ≤10-6 for the diffuse clouds along the line of sight and a few times 10-6 for the gas associated with the Sgr B2 envelope, in good agreement with water vapor observations from the Submillimeter Wave Astronomy Satellite.

ISO LWS observations of planetary nebula fine-structure lines

We have obtained 43–198 μm far-infrared (IR) spectra for a sample of 51 Galactic planetary nebulae (PN) and protoplanetary nebulae (PPN), using the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO). Spectra were also obtained of the former PN candidate Lo 14. The spectra yield fluxes for the fine-structure lines [N ii] 122 μm, [N iii] 57 μm and [O iii] 52 and 88 μm emitted in the ionized regions and the [O i] 63- and 146-μm and [C ii] 158-μm lines from the photodissociation regions (PDRs), which have been used to determine electron densities and ionic abundances for the ionized regions and densities, temperatures and gas masses for the PDRs. The strong [N iii] and [O iii] emission lines detected in the LWS spectrum taken centred on Lo 14 could be associated with the nearby strong radio and infrared source G 331.5–0.1. We find that the electron densities yielded by the [O iii] 88 μm/52 μm doublet ratio are systematically lower than those derived from the optical [Ar iv] λ4740/λ4711 and [Cl iii] λ5537/λ5517 doublet ratios, which have much higher critical densities than the 52- and 88-μm lines, suggesting the presence of density inhomogeneities in the nebulae. Ionic abundances, N+/H+,N2+/H+ and O2+/H+, as well as the N2+/O2+ abundance ratio, which provides a good approximation to the N/O elemental abundance ratio, are derived. Although ionic abundances relative to H+ deduced from the far-IR fine-structure lines are sensitive to the adopted electron density and the presence of density inhomogeneities, the strong dependence on the nebular physical conditions is largely cancelled out when N2+/O2+ is calculated from the 57 μm/(52 μm+88 μm) flux ratio, owing to the similarity of the critical densities of the lines involved. The temperatures and densities of the PDRs around 24 PN have been determined from the observed [O i] and [C ii] line intensity ratios. Except for a few objects, the deduced temperatures fall between 200 and 500 K, peaking around 250 K. The densities of the PDRs vary from 104–105 cm−3, reaching 3×105 cm−3 in some young compact PN. With a derived temperature of 1600 K and a density of 105 cm−3, the PDR of NGC 7027 is one of the warmest and at the same time one of the densest amongst the nebulae studied. For most of the PN studied, the [C ii]-emitting regions contain only modest amounts of material, with gas masses ≲0.1 M⊙. Exceptional large PDR masses are found for a few nebulae, including NGC 7027, the bipolar nebulae M2-9 and NGC 6302, the young dense planetary nebulae BD+30°3639, IC 418 and NGC 5315, and the old, probably recombining, nebulae IC 4406 and NGC 6072.

ISO LWS Observations of Mars—Detection of Rotational Modulation in the Far Infrared

A series of far infrared (FIR) spectra of Mars between 43 and 196 μm was taken with the Long Wavelength Spectrometer (LWS) on board the European Space Agency's Infrared Space Observatory (ISO). Analysis of these medium-resolution data reveals a rotational modulation of the disk-averaged FIR brightness temperature. The observed variability of ±1.5% in the LWS wavelength range compares favorably with the predictions of a thermophysical model of the martian surface. A surface emissivity of ∼0.90 gives good agreement between the measured and the modeled brightness temperatures and leads to an improved estimate of the LWS photometric calibration uncertainty.

O-bearing Molecules in Carbon-rich Proto-Planetary Objects.

We present Infrared Space Observatory long-wavelength spectrometer observations of the proto-planetary nebula CRL 618, a star evolving very fast toward the planetary nebula stage. In addition to the lines of 12CO, 13CO, HCN, and HNC, we report on the detection of H2O and OH emission together with the fine-structure lines of [O i] at 63 and 145 µm. The abundance of the latter three species relative to 12CO are 4x10-2, 8x10-4, and 4.5 (approximate value) in the regions where they are produced. We suggest that O-bearing species other than CO are produced in the innermost region of the circumstellar envelope. The UV photons from the central star photodissociate most of the molecular species produced in the asymptotic giant branch (AGB) phase and allow a chemistry dominated by standard ion-neutral reactions. They not only allow these reactions for the formation of O-bearing species but also modify the abundances of C-rich molecules like HCN and HNC for which we found an abundance ratio of approximately 1, which is much lower than in AGB stars. The molecular abundances in the different regions of the circumstellar envelope have been derived from radiative transfer models and from our knowledge of its physical structure.

Atomic Carbon in Arp 220

We present detections of the 3P1-3P0 fine-structure line of neutral carbon and of the J=3-2 rotational line of carbon monoxide in the ultraluminous infrared galaxy Arp 220, obtained at the Caltech Submillimeter Observatory. C I emission in Arp 220 is found to be at the expected level in relation to the strong far-infrared (FIR) dust emission and to the CO emission, whereas the C II emission seen by the Infrared Space Observatory long-wavelength spectrometer is very weak. The C I data confirm the presence of a large mass of molecular gas in the nucleus of Arp 220, corresponding to a high extinction in the visible (A -->v=1000). For this galaxy, the total cooling due to atomic carbon is comparable to the total cooling due to carbon monoxide. We use the observed C I/CO (1-0) and CO (3-2)/CO (1-0) emissivity ratios, together with the photon-dominated region models, to infer an average gas density of n(H -->2) ≈ 10 -->4 cm -->−3. We estimate the UV radiation field in this galaxy from the FIR continuum, and independently from hydrogen recombination lines, to be about a factor of 40,000 larger than in the solar vicinity (G -->0=4×10 -->4). The relative weakness of the C II fine-structure line is explained by the large gas density, the exceptionally strong UV field, and the large dust opacity. C II emission from high-redshift analogs of Arp 220 may not be as easy to detect as had been anticipated. However, both CO and C I lines will still be strong emitters.

Detection of Far-Infrared Water Vapor, Hydroxyl, and Carbon Monoxide Emissions from the Supernova Remnant 3C 391

We report the detection of shock-excited far-infrared emission of H2O, OH, and CO from the supernova remnant 3C 391, using the Infrared Space Observatory Long-Wavelength Spectrometer. This is the first detection of thermal H2O and OH emission from a supernova remnant. For two other remnants, W28 and W44, CO emission was detected, but OH was detected only in absorption. The observed H2O and OH emission lines arise from levels within ~400 K of the ground state, consistent with collisional excitation in warm, dense gas created after the passage of the shock front through the dense clumps in the preshock cloud. The postshock gas we observe has a density ~2×105 cm-3 and temperature 100-1000 K, and the relative abundances of CO:OH:H2O in the emitting region are 100:1:15 for a temperature of 200 K. The presence of a significant column of warm H2O suggests that the chemistry has been changed significantly by the shock. The existence of significant column densities of both OH and H2O, which is at odds with models for nondissociative shocks into dense gas, could be due to photodissociation of H2O or a mix of fast and slow shocks through regions with different preshock density.