Far-Infrared and Submillimeter Emission from Galactic and Extragalactic Photodissociation Regions
TL;DR: In this article, the effects of metallicity and cloud extinction on the predicted line intensities were examined for 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.
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 = 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.
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TL;DR: In this paper, the authors provided an analytic approximation for Pmin as a function of metallicity, far-ultraviolet radiation field, and the ionization rate of atomic hydrogen.
Abstract: Much of the interstellar medium in disk galaxies is in the form of neutral atomic hydrogen, H i. This gas can be in thermal equilibrium at relatively low temperatures, Td300 K (the cold neutral medium (CNM)), or at temperatures somewhat less than 10 4 K (the warm neutral medium (WNM)). These two phases can coexist over a narrow range of pressures, PminPPmax. We determine Pmin and Pmax in the plane of the Galaxy as a function of Galactocentric radius R using recent determinations of the gas heating rate and the gas-phase abundances of interstellar gas. We provide an analytic approximation for Pmin as a function of metallicity, far-ultraviolet radiation field, and the ionization rate of atomic hydrogen. Our analytic results show that the existence of Pmin, or the possibility of a two-phase equilibrium, generally requires that H + exceed C + in abundance at Pmin. The abundance of H + is set by EUV/soft X-ray photoionization and by recombination with negatively charged polycyclic aromatic hydrocarbons. In order to assess whether thermal or pressure equilibrium is a realistic assumption, we define a parameter � � tcool=tshock, where tcool is the gas cooling time and tshock is the characteristic shock time or '' time between shocks in a turbulent medium.'' For � < 1 gas has time to reach thermal balance between supernova-induced shocks. We find that this condition is satisfied in the Galactic disk, and thus the two-phase description of the interstellar H i is approximately valid even in the presence of interstellar turbulence. Observationally, the mean density nHi hi is often better determined than the local density, and we cast our results in terms of nHi hi as well. Over most of the disk of the Galaxy, the Hi must be in two phases: the weight of the Hi in the gravitational potential of the Galaxy is large enough to generate thermal pressures exceeding Pmin, so that turbulent pressure fluctuations can produce cold gas that is thermally stable; and the mean density of the H i is too low for the gas to be all CNM. Our models predict the presence of CNM gas to R ' 16 18 kpc, somewhat farther than previous estimates. An estimate for the typical thermal pressure in the Galactic plane for 3 kpcdRd18 kpc is Pth=k ' 1:4 � 10 4 expð� R=5:5 kpcÞ Kc m � 3 . At the solar circle, this gives Pth=k ' 3000 K cm � 3 . We show that this pressure is consistent with the C i*/C itot ratio observed by Jenkins & Tripp and the CNM temperature found by Heiles & Troland. We also examine the potential impact of turbulent heating on our results and provide parameterized expressions for the heating rate as a function of Galactic radius. Although the uncertainties are large, our models predict that including turbulent heating does not significantly change our results and that thermal pressures remain above Pmin to R ' 18 kpc. Subject headings: ISM: clouds — ISM: general — ISM: structure
1,110 citations
Cites methods from "Far-Infrared and Submillimeter Emis..."
...Using the photodissociation region models of Kaufman et al. (1999), modified by the chemistry and thermal processes discussed in this paper, we find that if the gas density and UV field illumination are fixed, the H i brightness temperature increases if the metallicity (and therefore the gasphase…...
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...We have also updated the gas-phase chemical reaction rates according to the list of Millar, Farquhar, & Willacy (1997) and modified the H2 formation and dissociation rate according to the discussion in Kaufman et al. (1999)....
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TL;DR: In this article, the authors theoretically model this dark mass and find that the fraction of the molecular mass in this dark component is remarkably constant (~0.3 for average visual extinction through the cloud ) and insensitive to the incident ultraviolet radiation field strength, the internal density distribution, and the mass of a molecular cloud as long as, or equivalently, the product of the average hydrogen nucleus column and the metallicity through a cloud, is constant.
Abstract: The mass of molecular gas in an interstellar cloud is often measured using line emission from low rotational levels of CO, which are sensitive to the CO mass, and then scaling to the assumed molecular hydrogen H2 mass. However, a significant H2 mass may lie outside the CO region, in the outer regions of the molecular cloud where the gas-phase carbon resides in C or C+. Here, H2 self-shields or is shielded by dust from UV photodissociation, whereas CO is photodissociated. This H2 gas is "dark" in molecular transitions because of the absence of CO and other trace molecules, and because H2 emits so weakly at temperatures 10 K
734 citations
Cites background from "Far-Infrared and Submillimeter Emis..."
...The first part is derived from Kaufman et al. (1999), Wolfire et al. (1990), and Tielens & Hollenbach (1985) models and is the main program for calculating the chemistry, thermal balance, and line emission....
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TL;DR: The Primordial Inflation Explorer (PIXIE) as mentioned in this paper is an Explorer-class mission to measure the gravity-wave signature of primordial inflation through its distinctive imprint on the linear polarization of the cosmic microwave background.
Abstract: The Primordial Inflation Explorer (PIXIE) is a concept for an Explorer-class mission to measure the gravity-wave signature of primordial inflation through its distinctive imprint on the linear polarization of the cosmic microwave background. The instrument consists of a polarizing Michelson interferometer configured as a nulling polarimeter to measure the difference spectrum between orthogonal linear polarizations from two co-aligned beams. Either input can view the sky or a temperature-controlled absolute reference blackbody calibrator. Rhe proposed instrument can map the absolute intensity and linear polarization (Stokes I, Q, and U parameters) over the full sky in 400 spectral channels spanning 2.5 decades in frequency from 30 GHz to 6 THz (1 cm to 50 micron wavelength). Multi-moded optics provide background-limited sensitivity using only 4 detectors, while the highly symmetric design and multiple signal modulations provide robust rejection of potential systematic errors. The principal science goal is the detection and characterization of linear polarization from an inflationary epoch in the early universe, with tensor-to-scalar ratio r < 10..3 at 5 standard deviations. The rich PIXIE data set can also constrain physical processes ranging from Big Bang cosmology to the nature of the first stars to physical conditions within the interstellar medium of the Galaxy.
688 citations
Cites background or methods from "Far-Infrared and Submillimeter Emis..."
...Figure 16 depicts how the intensity of the UV radiation field incident on the surface of the PDR, G0, can be used together with different line ratios to determine the elemental abundances and physical conditions of the PDRs that give rise to the line emission [54]....
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...Contour diagrams, taken from (author?) [54], illustrate the use of line ratios to determine the density n and UV flux intensity G0 in PDRs....
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TL;DR: In this article, a multiline UV pumping model was proposed to compare the effect of self-shielding on the overall fluorescent efficiency of the photodissociation front, including the effects of line overlap.
Abstract: The structure of stationary photodissociation fronts is revisited. H_2 self- shielding is discussed, including the effects of line overlap. We find that line overlap is important for N(H_2) > 10^{20} cm^{-2}. We compute multiline UV pumping models, and compare these with simple analytic approximations for the effects of self-shielding.
The overall fluorescent efficiency of the photodissociation front is obtained for different ratios of chi/n_H (where chi characterizes the intensity of the incident UV) and different dust extinction laws. The dust optical depth tau_{pdr} to the point where 50% of the H is molecular is found to be a simple function of a dimensionless quantity phi_0 depending on chi/n_H, the rate coefficient for H_2 formation on grains, and the UV dust opacity. The fluorescent efficiency of the PDR also depends primarily on phi_0 for chi 10^4K, but shows some sensitivity to the v-J distribution of newly-formed H_2. The 1-0S(1)/2-1S(1) and 2-1S(1)/6-4Q(1) intensity ratios, the ortho/para ratio, and the rotational temperature in the $v$=1 and $v$=2 levels are computed as functions of the temperature and density, for different values of chi and n_H.
We apply our models to the reflection nebula NGC 2023. We are best able to reproduce the observations with models having chi=5000, n_H=10^5 cm^{-3}.
548 citations
Max Planck Society1, National Radio Astronomy Observatory2, University of Maryland, College Park3, Ohio State University4, University of Toledo5, Princeton University6, McMaster University7, University of Massachusetts Amherst8, University of Cambridge9, University of Paris-Sud10, Stony Brook University11, Heidelberg University12, University of Hertfordshire13, ASTRON14, University of Wyoming15, University of Arizona16, Raytheon17, INAF18, Institut d'Astrophysique de Paris19, University of California, San Diego20, California Institute of Technology21, Leiden University22, Space Telescope Science Institute23, Louisiana State University24, DSM25, University of Cologne26
TL;DR: In this paper, the authors presented a large-scale spatial resolution map of the CO-to-H$2}$ conversion factor and dust-togas ratio (DGR) in 26 nearby, star-forming galaxies.
Abstract: We present ~{}kiloparsec spatial resolution maps of the CO-to-H$_{2}$ conversion factor ({$α$}$_{CO}$) and dust-to-gas ratio (DGR) in 26 nearby, star-forming galaxies. We have simultaneously solved for {$α$}$_{CO}$ and the DGR by assuming that the DGR is approximately constant on kiloparsec scales. With this assumption, we can combine maps of dust mass surface density, CO-integrated intensity, and H I column density to solve for both {$α$}$_{CO}$ and the DGR with no assumptions about their value or dependence on metallicity or other parameters. Such a study has just become possible with the availability of high-resolution far-IR maps from the Herschel key program KINGFISH, $^{12}$CO J = (2-1) maps from the IRAM 30 m large program HERACLES, and H I 21 cm line maps from THINGS. We use a fixed ratio between the (2-1) and (1-0) lines to present our {$α$}$_{CO}$ results on the more typically used $^{12}$CO J = (1-0) scale and show using literature measurements that variations in the line ratio do not affect our results. In total, we derive 782 individual solutions for {$α$}$_{CO}$ and the DGR. On average, {$α$}$_{CO}$ = 3.1 M $_{☉}$ pc$^{–2}$ (K km s$^{–1}$)$^{–1}$ for our sample with a standard deviation of 0.3 dex. Within galaxies, we observe a generally flat profile of {$α$}$_{CO}$ as a function of galactocentric radius. However, most galaxies exhibit a lower {$α$}$_{CO}$ value in the central kiloparsec{mdash}a factor of ~{}2 below the galaxy mean, on average. In some cases, the central {$α$}$_{CO}$ value can be factors of 5-10 below the standard Milky Way (MW) value of {$α$}$_{CO, MW}$ = 4.4 M $_{☉}$ pc$^{–2}$ (K km s$^{–1}$)$^{–1}$. While for {$α$}$_{CO}$ we find only weak correlations with metallicity, the DGR is well-correlated with metallicity, with an approximately linear slope. Finally, we present several recommendations for choosing an appropriate {$α$}$_{CO}$ for studies of nearby galaxies.
533 citations
Cites background from "Far-Infrared and Submillimeter Emis..."
...…shift relative to the HI/H2 transition due to a lack of dust shielding, leading to layers of “COdark” H2 (Tielens & Hollenbach 1985; van Dishoeck & Black 1988; Wolfire et al. 1993; Kaufman et al. 1999; Bell et al. 2006). αCO will increase when there are significant amounts of gas in these layers....
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3,700 citations
TL;DR: In this paper, a self-consistent model of the interstellar medium was developed to account for the observed pressure of interstellar clouds, the galactic soft X-ray background, the O VI absorption line observations, the ionization and heating of much of the galaxy, and the motions of the clouds.
Abstract: Supernova explosions in a cloudy interstellar medium produce a three-component medium in which a large fraction of the volume is filled with hot, tenuous gas. In the disk of the galaxy the evolution of supernova remnants is altered by evaporation of cool clouds embedded in the hot medium. Radiative losses are enhanced by the resulting increase in density and by radiation from the conductive interfaces between clouds and hot gas. Mass balance (cloud evaporation rate=dense shell formation rate) and energy balance (supernova shock input=radiation loss) determine the density and temperature of the hot medium with (n, T) = (10/sup -2.5/, 10/sup 5.7/) being representative values. Very small clouds will be rapidly evaporated or swept up. The outer edges of ''standard'' clouds ionized by the diffuse UV and soft X-ray backgrounds provide the warm (approx.10/sup 4/ K) ionized and neutral components. A self-consistent model of the interstellar medium developed herein accounts for the observed pressure of interstellar clouds, the galactic soft X-ray background, the O VI absorption line observations, the ionization and heating of much of the interstellar medium, and the motions of the clouds. In the halo of the galaxy, where the clouds are relatively unimportant, we estimate (n, T)more » = (10/sup -3.3/, 10/sup 6.0/) below one pressure scale height. Energy input from halo supernovae is probably adequate to drive a galactic wind.« less
1,980 citations
"Far-Infrared and Submillimeter Emis..." refers background in this paper
...In addition, the warm neutral medium (n ≃ 0.25 cm−3) of the Galaxy (c.f. McKee & Ostriker 1977; Wolfire et al. 1995), the neutral gas around planetary nebulae, and photodissociated winds from red giant or AGB stars are PDRs. Densities in PDRs range from n = 0.25 cm−3 to n ∼> 107 cm−3, and incident…...
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TL;DR: The high-resolution spectrograph of the Hubble Space Telescope (HST) has yielded precision abundance results for a range of interstellar environments, including gas in the local medium, in the warm neutral medium and in cold diffuse clouds, and in distant halo clouds.
Abstract: ▪ Abstract The Goddard High-Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST) has yielded precision abundance results for a range of interstellar environments, including gas in the local medium, in the warm neutral medium, in cold diffuse clouds, and in distant halo clouds. Through GHRS studies, investigators have determined the abundances of elements such as C, N, O, Mg, Si, S, and Fe in individual interstellar clouds. These studies have provided new information about the composition of interstellar dust grains, the origin of the Galactic high-velocity cloud system, and the processes that transport gas between the disk and the halo. Precision measurements of the interstellar D to H ratio and of the abundances of r- and s-process elements have also provided fiducial reference values for cosmological and stellar evolutionary observations and theoretical models.
1,304 citations
"Far-Infrared and Submillimeter Emis..." refers methods in this paper
...We use the cool diffuse cloud gas phase abundances from Savage & Sembach (1996) for our standard model....
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TL;DR: In this article, a theoretical parameter study of the temperature and chemical structure of dense photodissociation regions and their resultant spectrum is presented, which is relevant not only to the dust and gas between molecular clouds and H II regions, but also apply to any neutral cloud illuminated by intense FUV fluxes.
Abstract: A theoretical parameter study of the temperature and chemical structure of dense photodissociation regions and their resultant spectrum is presented. Models are discussed which are relevant not only to the dust and gas between molecular clouds and H II regions, but also apply to any neutral cloud illuminated by intense FUV fluxes. The models relate observed line and continuum emission from these regions to physical parameters such as the gas density and temperature, the elemental and chemical abundances, the local radiation field, and the grain properties. The results are applied to observational data from the OMC-1 region. The model shows that the observed high brightness temperature of the C I 609 microns line can be explained by emission from the C(+)/C/CO transition region. This difference with previous chemical models is due to a higher gas phase elemental abundance of carbon, to the charge exchange reactions of C(+) with S and SiO, and to carbon self-shielding.
1,234 citations
"Far-Infrared and Submillimeter Emis..." refers background or methods in this paper
...Besides changing the standard gas phase abundances (note in particular the decrease in the gas phase carbon abundance in Table 1 from the value used in WTH90), we have made a number of changes to the chemical, heating and cooling rate coefficients as compared with TH85, WTH90 and HTT91....
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...A series of models have been used to interpret these observations (e.g., Tielens & Hollenbach 1985a, hereafter TH85; van Dishoeck & Black 1986, 1988; Sternberg & Dalgarno 1989; Wolfire et al. 1990, hereafter WTH90; Hollenbach et al. 1991, hereafter HTT91; Abgrall et al. 1992; Le Bourlot et al.…...
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...As discussed in TH85, Tielens & Hollenbach (1985b) and HTT91, the column density of neutral carbon is insensitive to the strength of the external FUV field....
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...We have modeled the emission from photodissociation regions using the PDR code of TH85, but updated through the use of the most up to date values of atomic and molecular data, chemical rate coefficients, and grain photoelectric heating rates....
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TL;DR: In this paper, a photoelectric heating rate from small grains and polycyclic aromatic hydrocarbons (PAHs) was proposed to account for the size distribution of particles extending from 100 to 3 A radius.
Abstract: We calculate the thermal equilibrium gas temperature of the diffuse interstellar medium. Our method incorporates a new photoelectric heating rate from small grains and polycyclic aromatic hydrocarbons (PAHs) that accounts for a size distribution of particles extending from 100 to 3 A radius. We also include a detailed treatment of the ionization rates and heating due to the soft X-ray background and due to cosmic rays. Phase diagrams (thermal pressure P versus hydrogen density n) are presented for gas that is illuminated by local interstellar far-ultraviolet (FUV) and X-ray radiation fields. A stable two-phase medium is produced with thermal pressure in the range P/k approximately = to 10(exp 3-4) K/cc. We demonstrate that photoelectric heating from PAHs dominates in the warm neutral phase (WNM) and cold neutral phase (CNM). If the C II (158 micrometers cooling per hydrogen nucleus in the solar neighborhood represents an average value for the Galaxy, we predict L(sub CII) approximately = to 7 x 10(exp 7) solar luminosities from the CNM in the Galaxy, comparable to that observed by the Cosmic Background Explorer (COBE). We discuss the dependence of the results on absorbing column density, gas phase abundances, dust abundances and metallicity, FUV field, and the X-ray radiation field. These results will be useful in modeling the multiphase structure of high-velocity clouds in the halo, the interstellar matter (ISM) at other galactocentric radii, and the ISM in external galaxies and galactic nuclei.
969 citations
"Far-Infrared and Submillimeter Emis..." refers background or methods in this paper
...In addition, the warm neutral medium (n ≃ 0.25 cm−3) of the Galaxy (c.f. McKee & Ostriker 1977; Wolfire et al. 1995), the neutral gas around planetary nebulae, and photodissociated winds from red giant or AGB stars are PDRs. Densities in PDRs range from n = 0.25 cm−3 to n ∼> 107 cm−3, and incident…...
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...We use revised collision rates as discussed in Burton et al. (1990), Spaans et al. (1994), and Wolfire et al. (1995)....
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...We have also modified the cosmic ray heating and ionization terms using fits to the results of Shull & Van Steenberg (1985) (see Wolfire et al. 1995 for functions)....
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