Michael L. Luhman

Far-Infrared and Submillimeter Emission from Galactic and Extragalactic Photodissociation Regions

Photodissociation region (PDR) models are computed over a wide range of physical conditions, from those appropriate to giant molecular clouds illuminated by the interstellar radiation field to the conditions experienced by circumstellar disks very close to hot massive stars. These models use the most up-to-date values of atomic and molecular data, the most current chemical rate coefficients, and the newest grain photoelectric heating rates, which include treatments of small grains and large molecules. In addition, we examine the effects of metallicity and cloud extinction on the predicted line intensities. Results are presented for PDR models with densities over the range n = 101-107 cm-3 and for incident far-ultraviolet radiation fields over the range G0 = 10-0.5-106.5 (where G0 is the far-ultravioliet [FUV] flux in units of the local interstellar value), for metallicities Z = 1 and 0.1 times the local Galactic value, and for a range of PDR cloud sizes. We present line strength and/or line ratio plots for a variety of useful PDR diagnostics: [C II] 158 μm, [O I] 63 μm and 145 μm, [C I] 370 μm and 609 μm, CO J = 1-0, J = 2-1, J = 3-2, J = 6-5, and J = 15-14, as well as the strength of the far-infrared continuum. These plots will be useful for the interpretation of Galactic and extragalactic far-infrared and submillimeter spectra observable with the Infrared Space Observatory (ISO), the Stratospheric Observatory for Infrared Astronomy, the Submillimeter Wave Astronomy Satellite, the Far Infrared and Submillimeter Telescope, and other orbital and suborbital platforms. As examples, we apply our results to ISO and ground-based observations of M82, NGC 278, and the Large Magellanic Cloud. Our comparison of the conditions in M82 and NGC 278 show that both the gas density and FUV flux are enhanced in the starburst nucleus of M82 compared with those in the normal spiral NGC 278. We model the high [C II]/CO ratio observed in the 30 Doradus region of the LMC and find that it can be explained either by lowering the average extinction through molecular clouds or by enhancing the density contrast between the atomic layers of PDRs and the CO-emitting cloud cores. The ratio L[CO]/M[H2] implied by the low extinction model gives cloud masses too high for gravitational stability. We therefore rule out low-extinction clouds as an explanation for the high [C II]/CO ratio and instead appeal to density contrast in AV = 10 clouds.

The [C II] 158 Micron Line Deficit in Ultraluminous Infrared Galaxies Revisited

We present a study of the [C II] 157.74 μm fine-structure line in a sample of 15 ultraluminous infrared (IR) galaxies (IR luminosity LIR ≥ 1012 L☉; ULIRGs) using the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). We confirm the observed order of magnitude deficit (compared to normal and starburst galaxies) in the strength of the [C II] line relative to the far-infrared (FIR) dust continuum emission found in our initial report, but here with a sample that is twice as large. This result suggests that the deficit is a general phenomenon affecting 4 out of 5 ULIRGs. We present an analysis using observations of generally acknowledged photodissociation region (PDR) tracers ([C II], [O I] 63 and 145 μm, and FIR continuum emission), which suggests that a high ultraviolet flux G0 incident on a moderate density n PDR could explain the deficit. However, comparisons with other ULIRG observations, including CO (1-0), [C I] (1-0), and 6.2 μm polycyclic aromatic hydrocarbon (PAH) emission, suggest that high G0/n PDRs alone cannot produce a self-consistent solution that is compatible with all of the observations. We propose that non-PDR contributions to the FIR continuum can explain the apparent [C II] deficiency. Here, unusually high G0 and/or n physical conditions in ULIRGs as compared to those in normal and starburst galaxies are not required to explain the [C II] deficit. Dust-bounded photoionization regions, which generate much of the FIR emission but do not contribute significant [C II] emission, offer one possible physical origin for this additional non-PDR component. Such environments may also contribute to the observed suppression of FIR fine-structure emission from ionized gas and PAHs, as well as the warmer FIR colors found in ULIRGs. The implications for observations at higher redshifts are also revisited.

Infrared Space Observatory Measurements of a [C II] 158 micron Line Deficit in Ultraluminous Infrared Galaxies

We report measurements of the [C II] 157.74 μm fine-structure line in a sample of seven ultraluminous infrared galaxies (ULIGs) (LIR > 1012 L☉) with the Long Wavelength Spectrometer on the Infrared Space Observatory. The [C II] line is an important coolant in galaxies and arises in interstellar gas exposed to far-ultraviolet photons (hν≥11.26 eV); in ULIGs, this radiation stems from the bursts of star formation and/or from the active galactic nuclei that power the tremendous infrared luminosity. The [C II] 158 μm line is detected in four of the seven ULIGs; the absolute line flux (about a few times 10-20 W cm-2) represents some of the faintest extragalactic[C II] emission yet observed. Relative to the far-infrared continuum, the [C II] flux from the observed ULIGs is ~10% of that seen from nearby normal and starburst galaxies. We discuss possible causes for the [C II] deficit, namely (1) self-absorbed or optically thick [C II] emission, (2) saturation of the [C II] emission in photodissociated gas with high gas density n (3 × 103 cm-3) or with a high ratio of incident UV flux G0 to n (G0/n 10 cm3), or (3) the presence of a soft ultraviolet radiation field caused, for example, by a stellar population deficient in massive main-sequence stars. As nearby examples of colliding galaxies, ULIGs may resemble high-redshift protogalaxies in both morphology and spectral behavior. If true, the suggested [C II] deficit in ULIGs poses limitations on the detection rate of high-z sources and on the usefulness of [C II] as an eventual tracer of protogalaxies.

ISO LWS spectroscopy of M82: A unified evolutionary model

We present the first complete far-infrared spectrum (43-197 μm) of M82, the brightest infrared galaxy in the sky, taken with the Long Wavelength Spectrometer of the Infrared Space Observatory (ISO). We detected seven fine structure emission lines, [O I] 63 and 145 μm, [O III] 52 and 88 μm , [N II] 122 μm, [N III] 57 μm, and [C II] 158 μm, and fitted their ratios to a combination starburst and photodissociation region (PDR) model. The best fit is obtained with H II regions with n = 250 cm-3, an ionization parameter of 10-3.5, and PDRs with n = 103.3 cm-3 and a far-ultraviolet flux of G0 = 102.8. We applied both continuous and instantaneous starburst models, with our best fit being a 3-5 Myr old instantaneous burst model with a 100 M⊙ cutoff. We also detected the ground-state rotational line of OH in absorption at 119.4 μm. No excited level OH transitions are apparent, indicating that the OH is almost entirely in its ground state with a column density ∼4 × 1014 cm-2. The spectral energy distribution over the long-wavelength spectrometer wavelength range is well fitted with a 48 K dust temperature and an optical depth, τDust ∝ λ-1.

ISO Far-IR spectroscopy of IR-bright galaxies and ULIRGs

Based on far-infrared spectroscopy of a small sample of nearbyinfrared-bright and ultraluminous infrared galaxies (ULIRGs) with theISO Long Wavelength Spectrometer we find adramatic progression in ionic/atomic fine-structure emission line andmolecular/atomic absorption line characteristics in these galaxiesextending from strong [O III]52,88 μm and [N III]57 μm lineemission to detection of only faint [C II]158 μm line emissionfrom gas in photodissociation regions in the ULIRGs. The molecularabsorption spectra show varying excitation as well, extending fromgalaxies in which the molecular population mainly occupies the groundstate to galaxies in which there is significant population in higherlevels. In the case of the prototypical ULIRG, the merger galaxy Arp220, the spectrum is dominated by absorption lines of OH, H2O, CH,and [O I]. Low [O III]88 μm line flux relative to the integratedfar-infrared flux correlates with low excitation and does not appear tobe due to far-infrared extinction or to density effects. A progressiontoward soft radiation fields or very dusty H II regions may explainthese effects.

Near-Infrared Spectroscopy of Photodissociation Regions: The Orion Bar and Orion S

We have obtained moderate-resolution (R ~ 3000) spectra of the Orion bar and Orion S regions at J (1.25 μm), H (1.64 μm), and K (2.2 μm). Toward the bar, the observations reveal a large number of H2 emission lines that, when compared to model predictions of Draine & Bertoldi, are indicative of a high-density photodissociation region (PDR) (nH = 106 cm-3, χ = 105, T0 = 1000 K) rather than of shocked material. Behind the bar and into the molecular cloud, the H2 spectrum again matches well with that predicted for a dense PDR (nH = 106 cm-3) but with a lower temperature (T0 = 500 K) and UV field strength (χ = 104). The H2 spectrum and stratification of near-IR emission lines (O I, H I, [Fe II], [Fe III], H2) near Orion S imply the presence of a dense PDR with an inclined geometry in this region (nH = 106 cm-3, χ = 105, T0 = 1500 K). The extinction measurements toward the bar (AK ~ 2.6) and Orion S (AK ~ 2.1) H2 emission regions are much larger than expected from either face-on (AK ~ 0.1) or edge-on (AK ~ 1) homogeneous PDRs, indicating that clumps may significantly affect the structure of the PDRs. In addition, we have observed the strongest ~30 near-IR He I emission lines, many of which have not been detected previously. There is good agreement between most observed and theoretical He I line ratios, while a few transitions with upper levels of n3P (particularly 43P-33S 1.2531 μm) are enhanced over strengths expected from collisional excitation. This effect is possibly due to opacity in the UV series n3P-23S. We also detect several near-IR [Fe II] and [Fe III] transitions with line ratios indicative of low densities (ne ~ 103-104 cm-3), whereas recent observations of optical [Fe II] emission imply the presence of high-density gas (ne ~ 106 cm-3). These results are consistent with a model in which high-density, partially-ionized gas is the source of the iron transitions observed in the optical, while low-density, fully-ionized material is responsible for the near-IR emission lines.

Near-Infrared Spectra of IC 59/63 and NGC 1535: Comparing Infrared and Ultraviolet Observations of H2

We present observations of near-infrared H2 line emission toward the reflection/emission nebulae, IC 59 and IC 63, and the planetary nebula, NGC 1535. Each source has been observed previously in the ultraviolet, where H2 was detected in emission toward IC 63 and in absorption toward NGC 1535. In IC 63, we have detected the 1.601 ?m v = 6-4 Q(1), 2.121 ?m v = 1-0 S(1), and 2.247 ?m v = 2-1 S(1) lines of H2 arising from a near-infrared fluorescent cascade following ultraviolet continuum pumping. The detection marks the first time that both infrared and ultraviolet portions of the H2 fluorescent cascade have been measured in a region exposed to far-ultraviolet continuum photons. Furthermore, we also report 1-0 S(1) and 2-1 S(1) fluorescent emission toward IC 59, a source previously thought to display no H2 fluorescence and considered devoid of molecules based on ultraviolet and CO observations. Toward NGC 1535, we find no H2 emission in the near-infrared, in spite of the reported ultraviolet H2 absorption.