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

Abstract

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.

ALMA observations of atomic carbon inz ∼ 4 dusty star-forming galaxies

“…It can be seen that the UV field strengths derived by the two models are broadly comparable, with no systematic differences (albeit with relatively large scatter). The densities, however, show a marked difference; the densities derived by 3d-pdr are systematically higher than those derived by Kaufman et al (1999). The mean derived density produced by the Kaufman et al (1999) models is < log n >= 4.4 ± 0.4 cm 3 , a factor of ∼ 6 lower than that produced by 3d-pdr (< log n >= 5.2 ± 0.6 cm 3 ).…”

“…As the J transition increases, the peak of local emissivity occurs at lower and lower AV, which can then provide information about the state of PDRs in these conditions. Previous work presenting PDR analyses of molecular and atomic emission lines in DSFGs (e.g., Danielson et al 2011;Alaghband-Zadeh et al 2013) have generally used the well-known Kaufman et al (1999) PDR models. There are, however, a number of critical parameters that affect the PDR modelling results, which the Kaufman et al (1999) models do not account for.…”

“…It is therefore illuminating to compare the results produced by 3d-pdr with those produced by the older Kaufman et al (1999) models. Our method for deriving densities and UV field strengths from the Kaufman et al (1999) models is identical to the method we use for 3d-pdr; we produce contour plots based on the line ratios used above ([Cii]/ [Ci](1 − 0), CO(4 − 3)/CO(2 − 1), and [Ci](1 − 0)/CO(4 − 3)), and take log n and log G to be the peak of the probability distribution function defined by these line ratio constraints. Fig.…”

The nature of the [C ii] emission in dusty star-forming galaxies from the SPT survey

“…Graf et al 2012;Ossenkopf et al 2013), or by theoretical modelling (e.g. Kaufman et al 1999). These studies obtain T (gas) = 100 − 500 K, which from Fig.…”

“…The intensities are modeled for a parameter space of the FUV field strength (G0) in units of the local Galactic interstellar radiation field (the 'Habing Field', 1.6 × 10 −3 ergs cm −3 s −1 ; Kaufman et al 1999) in the range 10 −0.5 G0 10 6.5 and the gas densities in the range 10 cm −3 n 10 7 cm −3 . The emission from different species depends on the density, the field strength and the depth into the cloud.…”