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 = 10 1 − 10 7 cm −3 and for incident far-ultraviolet radiation fields over the range G 0 = 10 −0.5 − 10 6.5 (where G 0 is the 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] 158µm, [O I] 63µm and 145µm, [C I] 370µm and 609µm, 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 1 NRC-NRL Research Associate extragalactic far infrared and submillimeter spectra observable with the Infrared Space Observatory, 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 Magellenic 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 the normal spiral NGC 278. We model the high [C II]/CO ratio observed in the 30 Doradus region of the LMC and find it can be explained either by lowering the average extinction through molecular clouds or by enhancing the density contrast between the atomic layers of PDR and the CO emitting cloud cores. The ratio L[CO]/M[H 2 ] 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 A V = 10 clouds.

We present a study of the [C ii] 157.74 lm fine-structure line in a sample of 15 ultraluminous infrared (IR) galaxies (IR luminosity L IR k10 12 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 lm, and FIR continuum emission), which suggests that a high ultraviolet flux G 0 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 lm polycyclic aromatic hydrocarbon (PAH) emission, suggest that high G 0 =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 G 0 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.

We present the first complete far-infrared spectrum (43 to 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 fit their ratios to a combination starburst and photo-dissociation region (PDR) model. The best fit is obtained with HII regions with n = 250 cm −3 and an ionization parameter of 10 −3.5 and PDRs with n = 10 3.3 cm −3 and a far-ultraviolet flux of G o = 10 2.8 . 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 ⊙ cut-off. 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 -2entirely in its ground state with a column density ∼4x10 14 cm −2 . The spectral energy distribution over the LWS wavelength range is well fit with a 48 K dust temperature and an optical depth, τ Dust ∝ λ −1 .