Photoelectric Heating Efficiency Research Articles

C II] 158 μm line emission from Orion A

Context. The [C II] 158 μm fine-structure line is the dominant cooling line of moderate-density photodissociation regions (PDRs) illuminated by moderately bright far-ultraviolet (FUV) radiation fields. This makes this line a prime diagnostic for extended regions illuminated by massive stars. Aims. We aim to understand the origin of [C II] emission and its relation to other tracers of gas and dust in PDRs. One focus is a study of the heating efficiency of interstellar gas as traced by the [C II] line to test models of the photoelectric heating of neutral gas by polycyclic aromatic hydrocarbon (PAH) molecules and very small grains. Methods. We make use of a one-square-degree map of velocity-resolved [C II] line emission toward the Orion Nebula complex, and split this out into the individual spatial components, the expanding Veil Shell, the surface of OMC4, and the PDRs associated with the compact H II region of M43 and the reflection nebula NGC 1977. We employed Herschel far-infrared photometric images to determine dust properties. Moreover, we compared with Spitzer mid-infrared photometry to trace hot dust and large molecules, and velocity-resolved IRAM 30m CO(2–1) observations of the molecular gas. Results. The [C II] intensity is tightly correlated with PAH emission in the IRAC 8 μm band and far-infrared emission from warm dust, with small variations between the four studied subregions (Veil Shell, OMC4, M43, and NGC 1977). The correlation between [C II] and CO(2-1) is very different in the four subregions and is very sensitive to the detailed geometry of the respective regions. Constant-density PDR models are able to reproduce the observed [C II], CO(2–1), and integrated far-infrared (FIR) intensities. The physical conditions in the Veil Shell of the Orion Nebula, M43, and NGC 1977 reveal a constant ratio of thermal pressure pth over incident FUV radiation field measured by G0. We observe strong variations in the photoelectric heating efficiency in the Veil Shell behind the Orion Bar and these variations are seemingly not related to the spectral properties of the PAHs. Conclusions. The [C II] emission from the Orion Nebula complex stems mainly from moderately illuminated PDR surfaces. The correlations of the different tracers ([C II], FIR, CO, 70 μm, and 8 μm emission) show small variations that are not yet understood. Future observations with the James Webb Space Telescope can shine light on the PAH properties that may be linked to these variations.

C ii] Map of the Molecular Ring and Arms of the Spiral Galaxy NGC 7331*

Abstract We present the [C ii] 157.7 μm map of galaxy NGC 7331 obtained with the Field-Imaging Far-Infrared Line Spectrometer on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). This map extends an existent Herschel/Photodetector Array Camera and Spectrometer observation of the central strip of the galaxy to encompass the entire molecular ring and much of the disk, including multiple spiral arms with intense far-IR emission. We also present Herschel archival data of the [N ii] 205 μm line, which covers a substantial part of the [C ii] SOFIA observations and allows us to estimate the neutral fraction of the [C ii] emission along the ring and disk of the galaxy. We find that the neutral fraction rises with the distance from the center. In addition, by tracing the azimuthal variation of the neutral fraction, we are able to see how our observing perspective affects this measurement. The high inclination of NGC 7331 allows us to glimpse the internal walls of the molecular ring. There, young bright stars emit UV radiation causing more [C ii] emission to be produced in the ionized gas. On the outer walls, opaque dust shrouds the rest of the ring, making the neutral medium the dominant source of [C ii] emission. Through spatial analysis comparing the [C ii] emission to tracers of gas heating, we are able to investigate how the photoelectric heating efficiency varies throughout NGC 7331 and extend global measurements of the [C ii] deficit to local environments. Since the origin of [C ii] emission has typically been studied in face-on galaxies, our results shed a new light on the interpretation of [C ii] emission, especially when studying distant galaxies with unknown inclination.

Herschel spectroscopy of massive young stellar objects in the Magellanic Clouds

ABSTRACT We present Herschel Space Observatory Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver Fourier Transform Spectrometer (SPIRE FTS) spectroscopy of a sample of 20 massive Young Stellar Objects (YSOs) in the Large and Small Magellanic Clouds (LMC and SMC). We analyse the brightest far-infrared (far-IR) emission lines, that diagnose the conditions of the heated gas in the YSO envelope and pinpoint their physical origin. We compare the properties of massive Magellanic and Galactic YSOs. We find that [O i] and [C ii] emission, that originates from the photo-dissociation region associated with the YSOs, is enhanced with respect to the dust continuum in the Magellanic sample. Furthermore the photoelectric heating efficiency is systematically higher for Magellanic YSOs, consistent with reduced grain charge in low metallicity environments. The observed CO emission is likely due to multiple shock components. The gas temperatures, derived from the analysis of CO rotational diagrams, are similar to Galactic estimates. This suggests a common origin to the observed CO excitation, from low-luminosity to massive YSOs, both in the Galaxy and the Magellanic Clouds. Bright far-IR line emission provides a mechanism to cool the YSO environment. We find that, even though [O i], CO, and [C ii] are the main line coolants, there is an indication that CO becomes less important at low metallicity, especially for the SMC sources. This is consistent with a reduction in CO abundance in environments where the dust is warmer due to reduced ultraviolet-shielding. Weak H2O and OH emission is detected, consistent with a modest role in the energy balance of wider massive YSO environments.

Thermal Phases of the Neutral Atomic Interstellar Medium from Solar Metallicity to Primordial Gas

Abstract We study the thermal structure of the neutral atomic (H i) interstellar medium across a wide range of metallicities, from supersolar down to vanishing metallicity, and for varying UV intensities and cosmic-ray (CR) ionization rates. We calculate self-consistently the gas temperature and species abundances (with a special focus on the residual H2), assuming a thermal and chemical steady state. For solar metallicity, Z′ ≡ 1, we recover the known result that there exists a pressure range over which the gas is multiphased, with the warm (∼104 K, warm neutral medium (WNM)) and cold (∼100 K, cold neutral medium (CNM)) phases coexisting at the same pressure. At a metallicity Z′ ≈ 0.1, the CNM is colder (compared to Z′ = 1) due to the reduced efficiency of photoelectric heating. For Z′ ≲ 0.1, CR ionization becomes the dominant heating mechanism and the WNM-to-CNM transition shifts to ever-increasing pressure/density as the metallicity is reduced. For metallicities Z′ ≲ 0.01, H2 cooling becomes important, lowering the temperature of the WNM (down to ≈600 K), and smoothing out the multiphase phenomenon. At vanishing metallicities, H2 heating becomes effective and the multiphase phenomenon disappears entirely. We derive analytic expressions for the critical densities for the warm-to-cold phase transition in the different regimes, and the critical metallicities for H2 cooling and heating. We discuss potential implications on the star formation rates of galaxies and self-regulation theories.

Carbon radio recombination lines from gigahertz to megahertz frequencies towards Orion A

Context. The combined use of carbon radio recombination lines (CRRLs) and the 158μm-[CII] line is a powerful tool for the study of the energetics and physical conditions (e.g., temperature and density) of photodissociation regions (PDRs). However, there are few observational studies that exploit this synergy. Aims. Here we explore the relation between CRRLs and the 158μm-[CII] line in light of new observations and models. Methods. We present new and existing observations of CRRLs in the frequency range 0.15–230 GHz with ALMA, VLA, the GBT, Effelsberg 100m, and LOFAR towards Orion A (M 42). We complement these observations with SOFIA observations of the 158μm-[CII] line. We studied two PDRs: the foreground atomic gas, known as the Veil, and the dense PDR between the HII region and the background molecular cloud. Results. In the Veil we are able to determine the gas temperature and electron density, which we use to measure the ionization parameter and the photoelectric heating efficiency. In the dense PDR, we are able to identify a layered PDR structure at the surface of the molecular cloud to the south of the Trapezium cluster. There we find that the radio lines trace the colder portion of the ionized carbon layer, the C+/C/CO interface. By modeling the emission of the 158μm-[CII] line and CRRLs as arising from a PDR we derive a thermal pressure >5 × 107 K cm−3 and a radiation field G0 ≈ 105 close to the Trapezium. Conclusions. This work provides additional observational support for the use of CRRLs and the 158μm-[CII] line as complementary tools to study dense and diffuse PDRs, and highlights the usefulness of CRRLs as probes of the C+/C/CO interface.

SHINING, A Survey of Far-infrared Lines in Nearby Galaxies. II. Line-deficit Models, AGN Impact, [C ii]–SFR Scaling Relations, and Mass–Metallicity Relation in (U)LIRGs

Abstract The SHINING survey offers a great opportunity to study the properties of the ionized and neutral media of galaxies from prototypical starbursts and active galactic nuclei (AGNs) to heavily obscured objects. Based on Herschel/PACS observations of the main far-infrared (FIR) fine-structure lines, in this paper, we analyze the physical mechanisms behind the observed line deficits in galaxies, the apparent offset of luminous infrared galaxies (LIRGs) from the mass–metallicity relation, and the scaling relations between [C ii] 158 μm line emission and star formation rate (SFR). Based on a toy model and the Cloudy code, we conclude that the increase in the ionization parameter with FIR surface brightness can explain the observed decrease in the line-to-FIR continuum ratio of galaxies. In the case of the [C ii] line, the increase in the ionization parameter is accompanied by a reduction in the photoelectric heating efficiency and the inability of the line to track the increase in the FUV radiation field as galaxies become more compact and luminous. In the central approximately kiloparsec regions of AGN galaxies, we observe a significant increase in the [O i] 63 μm/[C ii] line ratio; the AGN impact on the line-to-FIR ratios fades on global scales. Based on extinction-insensitive metallicity measurements of LIRGs, we confirm that they lie below the mass–metallicity relation, but the offset is smaller than those reported in studies that use optical-based metal abundances. Finally, we present scaling relations between [C ii] emission and SFR in the context of the main sequence of star-forming galaxies.

The Survey of Lines in M31 (SLIM): The Drivers of the [C ii]/TIR Variation

Abstract The ratio of the [C ii] emission line over the total infrared emission (TIR) is often used as a proxy for the photoelectric (PE) heating efficiency ( ) of the far-ultraviolet (FUV) photons absorbed by dust in the interstellar medium. In the nearby galaxy M31, we measure a strong radial variation of [C ii]/TIR that we rule out as being due to an intrinsic variation in . [C ii]/TIR fails as a proxy for , because the TIR measures all dust heating, not just the contribution from FUV photons capable of ejecting electrons from dust grains. Using extensive multi-wavelength coverage from the FUV to far-infrared, we infer the attenuated FUV emission ( ), and the total attenuated flux ( ). We find [C ii]/TIR to be strongly correlated with / , indicating that, in M31 at least, one of the dominant drivers for [C ii]/TIR variation is the relative hardness of the absorbed stellar radiation field. We define , [C ii]/ which should be more closely related to the actual PE efficiency, which we find to be essentially constant ( ) in all explored fields in M31. This suggests that part of the observed variation of [C ii]/TIR in other galaxies is likely due to a change in the relative hardness of the absorbed stellar radiation field, caused by a combination of variations in the stellar population, dust opacity, and galaxy metallicity, though PE efficiency may also vary across a wider range of environments.

Neutral gas heating by X-rays in primitive galaxies: Infrared observations of the blue compact dwarf I Zw 18 with Herschel

Context. The neutral interstellar medium of galaxies acts as a reservoir to fuel star formation. The dominant heating and cooling mechanisms in this phase are uncertain in extremely metal-poor star-forming galaxies. The low dust-to-gas mass ratio and low polycyclic aromatic hydrocarbon abundance in such objects suggest that the traditional photoelectric effect heating may not be effective. Aims. Our objective is to identify the dominant thermal mechanisms in one such galaxy, I Zw 18 (1/30Z⊙), assess the diagnostic value of fine-structure cooling lines, and estimate the molecular gas content. Even though molecular gas is an important catalyst and tracer of star formation, constraints on the molecular gas mass remain elusive in the most metal-poor galaxies. Methods. Building on a previous photoionization model describing the giant H ii region of I Zw 18-NW within a multi-sector topology, we provide additional constraints using, in particular, the [C ii] 157 μm and [O i] 63 μm lines and the dust mass recently measured with the Herschel Space Telescope. Results. The heating of the H i region appears to be mainly due to photoionization by radiation from a bright X-ray binary source, while the photoelectric effect is negligible. Significant cosmic ray heating is not excluded. Inasmuch as X-ray heating dominates in the H i gas, the infrared fine-structure lines provide an average X-ray luminosity of order 4 × 1040 erg s-1 over the last few 104 yr in the galaxy. The upper limits to the [Ne v] lines provide strong constraints on the soft X-ray flux arising from the binary. A negligible mass of H2 is predicted. Nonetheless, up to ~107 M⊙ of H2 may be hidden in a few sufficiently dense clouds of order ≲5 pc (≲0.05′′) in size. Regardless of the presence of significant amounts of H2 gas, [C ii] and [O i] do not trace the so-called “CO-dark gas”, but they trace the almost purely atomic medium. Although the [C ii]+[O i] to total infrared ratio in I Zw 18 is similar to values in more metal-rich sources (~1%), it cannot be safely used as a photoelectric heating efficiency proxy. This ratio seems to be kept stable owing to a correlation between the X-ray luminosity and the star formation rate. Conclusions. X-ray heating could be an important process in extremely metal-poor sources. The lack of photoelectric heating due to the low dust-to-gas ratio tends to be compensated for by the larger occurrence and power of X-ray binaries in low-metallicity galaxies. We speculate that X-ray heating may quench star formation.

SOFIA/GREAT [C ii] observations in nearby clouds near the lines of sight towards B0355+508 and B0212+735

We present SOFIA/GREAT [C ii] 2 P 3/2 → 2 P 1/2 (1.9005369 THz) observations of nearby clouds near the lines of sight towards the quasars B0355+508 and B0212+735. These clouds have previously been identified as warm non-LTE diffuse clouds with a temperature of T ≳ 30 K and sub-thermally excited CO lines. They are highly structured in CO with small-scale bright spots ( I (CO J = 1–0)~ 5–20 K km s -1 ), both spectrally and spatially. This small-scale structure has been interpreted as small-scale variations in the chemistry, not as density and column-density structure. We did not detect [C ii] 158 μ m emission within the rms noise of ~0.1–0.3 K. Our non-detection in [C ii] contradicts the above scenario. In diffuse clouds, the efficiency of photo-electric heating is highest. Under the assumption that [C ii] is the dominant coolant in diffuse clouds, we can calculate the predicted [C ii] emission arising from these clouds. Based on the derived hydrogen column densities for the diffuse clouds, a line width in [C ii] of similar order to that of CO, and when at least the minimum amount of heating in the clouds is due to cosmic-ray heating, [C ii] line intensities ≳1.5 to 5 K km s -1 are expected, which is a factor ~3 to 15 above the upper limits of the observations. The upper limits of the [C ii] and the observed CO line intensities are, however, consistent with the intensities predicted for photon-dominated region (PDR) surfaces on regular cold ( T ~ 15 K) low-density cloud fragments. Lower temperatures lead to less excited [C ii]. The assumption of lower densities, which would equally lower the [C ii] excitation, contradicts the observed cloud sizes and column densities. The CO(2–1)/CO(1–0) line ratios observed in these clouds are consistent with cloud temperatures of T ~ 15 K. The KOSMA- τ PDR-model of a cold moderate-density clump or an ensemble of such clumps with a canonical mass-size relation and mass spectrum, consistent with the total column densities derived for low density, shows that cold PDRs can reproduce the observed CO intensities, the observed CO(1–0) and CO(2–1) ratio, and the observed upper limits for [C ii].

HERSCHEL EXTREME LENSING LINE OBSERVATIONS: [C ii] VARIATIONS IN GALAXIES AT REDSHIFTS z = 1–3*

ABSTRACT We observed the [C ii] line in 15 lensed galaxies at redshifts 1 < z < 3 using HIFI on the Herschel Space Observatory and detected 14/15 galaxies at 3σ or better. High magnifications enable even modestly luminous galaxies to be detected in [C ii] with Herschel. The [C ii] luminosity in this sample ranges from 8 × 107 L ⊙ to 3.7 × 109 L ⊙ (after correcting for magnification), confirming that [C ii] is a strong tracer of the ISM at high redshifts. The ratio of the [C ii] line to the total far-infrared (FIR) luminosity serves as a measure of the ratio of gas to dust cooling and thus the efficiency of the grain photoelectric heating process. It varies between 3.3% and 0.09%. We compare the [C ii]/FIR ratio to that of galaxies at z = 0 and at high redshifts and find that they follow similar trends. The [C ii]/FIR ratio is lower for galaxies with higher dust temperatures. This is best explained if increased UV intensity leads to higher FIR luminosity and dust temperatures, but gas heating does not rise due to lower photoelectric heating efficiency. The [C ii]/FIR ratio shows weaker correlation with FIR luminosity. At low redshifts highly luminous galaxies tend to have warm dust, so the effects of dust temperature and luminosity are degenerate. Luminous galaxies at high redshifts show a range of dust temperatures, showing that [C ii]/FIR correlates most strongly with dust temperature. The [C ii] to mid-IR ratio for the HELLO sample is similar to the values seen for low-redshift galaxies, indicating that small grains and PAHs dominate the heating in the neutral ISM, although some of the high [CII]/FIR ratios may be due to turbulent heating.

THE ORION H ii REGION AND THE ORION BAR IN THE MID-INFRARED

ABSTRACT We present mid-infrared photometry of the Orion bar obtained with the Faint Object infraRed Camera for the SOFIA Telescope (FORCAST) on board SOFIA at 6.4, 6.6, 7.7, 19.7, 31.5, and 37.1 μm. By complementing this observation with archival FORCAST and Herschel/PACS images, we are able to construct a complete infrared spectral energy distribution of the Huygens region in the Orion nebula. Comparing the infrared images with gas tracers, we find that PACS maps trace the molecular cloud, while the FORCAST data trace the photodissociation region (PDR) and the H ii region. Analysis of the energetics of the region reveal that the PDR extends for 0.28 pc along the line of sight and that the bar is inclined at an angle of 4°. The infrared and submillimeter images reveal that the Orion bar represents a swept-up shell with a thickness of 0.1 pc. The mass of the shell implies a shock velocity of ≃3 km s−1 and an age of ≃105 years for the H ii region. Our analysis shows that the UV and infrared dust opacities in the H ii region and the PDR are a factor 5 to 10 lower than in the diffuse interstellar medium. In the ionized gas, Lyα photons are a major source of dust heating at distances larger than ≃0.06 pc from θ 1 Ori C. Dust temperatures can be explained if the size of the grains is between 0.1 and 1 μm. We derive the photoelectric heating efficiency of the atomic gas in the Orion bar. The results are in good qualitative agreement with models and the quantitative differences indicate a decreased polycyclic aromatic hydrocarbon abundance in this region.

Insights into gas heating and cooling in the disc of NGC 891 fromHerschelfar-infrared spectroscopy

We present Herschel PACS and SPIRE spectroscopy of the most important far-infrared cooling lines in the nearby edge-on spiral galaxy, NGC 891: [CII] 158 $\mu$m, [NII] 122, 205 $\mu$m, [OI] 63, 145 $\mu$m, and [OIII] 88 $\mu$m. We find that the photoelectric heating efficiency of the gas, traced via the ([CII]+[OII]63)/$F_{\mathrm{TIR}}$ ratio, varies from a mean of 3.5$\times$10$^{-3}$ in the centre up to 8$\times$10$^{-3}$ at increasing radial and vertical distances in the disc. A decrease in ([CII]+[OII]63)/$F_{\mathrm{TIR}}$ but constant ([CII]+[OI]63)/$F_{\mathrm{PAH}}$ with increasing FIR colour suggests that polycyclic aromatic hydrocarbons (PAHs) may become important for gas heating in the central regions. We compare the observed flux of the FIR cooling lines and total IR emission with the predicted flux from a PDR model to determine the gas density, surface temperature and the strength of the incident far-ultraviolet (FUV) radiation field, $G_{0}$. Resolving details on physical scales of ~0.6 kpc, a pixel-by-pixel analysis reveals that the majority of the PDRs in NGC 891's disc have hydrogen densities of 1 < log ($n$/cm$^{-3}$) < 3.5 experiencing an incident FUV radiation field with strengths of 1.7 < log $G_0$ < 3. Although these values we derive for most of the disc are consistent with the gas properties found in PDRs in the spiral arms and inter-arm regions of M51, observed radial trends in $n$ and $G_0$ are shown to be sensitive to varying optical thickness in the lines, demonstrating the importance of accurately accounting for optical depth effects when interpreting observations of high inclination systems. With an empirical relationship between the MIPS 24 $\mu$m and [NII] 205 $\mu$m emission, we estimate an enhancement of the FUV radiation field strength in the far north-eastern side of the disc.

Gas and dust cooling along the major axis of M 33 (HerM33es)

We aim to better understand the heating of the gas by observing the prominent gas cooling line [CII] at 158um in the low-metallicity environment of the Local Group spiral galaxy M33 at scales of 280pc. In particular, we aim at describing the variation of the photoelectric heating efficiency with galactic environment. In this unbiased study, we used ISO/LWS [CII] observations along the major axis of M33, in combination with Herschel PACS and SPIRE continuum maps, IRAM 30m CO 2-1 and VLA HI data to study the variation of velocity integrated intensities. The ratio of [CII] emission over the far-infrared continuum is used as a proxy for the heating efficiency, and models of photon-dominated regions are used to study the local physical densities, FUV radiation fields, and average column densities of the molecular clouds. The heating efficiency stays constant at 0.8% in the inner 4.5kpc radius of the galaxy where it starts to increase to reach values of ~3% in the outskirts at about 6kpc radial distance. The rise of efficiency is explained in the framework of PDR models by lowered volume densities and FUV fields, for optical extinctions of only a few magnitudes at constant metallicity. In view of the significant fraction of HI emission stemming from PDRs, and for typical pressures found in the Galactic cold neutral medium (CNM) traced by HI emission, the CNM contributes ~15% to the observed [CII] emission in the inner 2kpc radius of M33. The CNM contribution remains largely undetermined in the south, while positions between 2 and 7.3kpc radial distance in the north of M33 show a contribution of ~40%+-20%.

Probing the role of polycyclic aromatic hydrocarbons in the photoelectric heating within photodissociation regions

We observationally investigate the relation between the photoelectric heating efficiency in PDRs and the charge of PAHs, which are considered to play a key role in photoelectric heating. Using PACS onboard Herschel, we observed six PDRs spanning a wide range of FUV radiation fields (G_0=100-10^5). To measure the photoelectric heating efficiency, we obtained the intensities of the main cooling lines, i.e., the [OI]63um, 145um, and [CII]158um, as well as the FIR continuum intensity. We used Spitzer/IRS spectroscopic mapping observations to investigate the MIR PAH features in the same regions. We decomposed the MIR PAH emission into that of neutral (PAH^0) and positively ionized (PAH^+) species to derive the fraction of the positively charged PAHs, and compare it to the photoelectric heating efficiency. The heating efficiency traced by ([OI]63um+[OI]145um+[CII]158um) / TIR, ranges between 0.1% and 0.9% in different sources, and the fraction of PAH^+ relative to (PAH^0 + PAH^+) spans from 0(+11)% to 87(+/-10)%. All positions with a high PAH^+ fraction show a low heating efficiency, and all positions with a high heating efficiency have a low PAH^+ fraction, supporting the scenario in which a positive grain charge results in a decreased heating efficiency. Theoretical estimates of the photoelectric heating efficiency show a stronger dependence on the charging parameter gamma=G_0 T^{1/2}/n_e than the observed efficiency reported in this study, and the discrepancy is significant at low gamma. The photoelectric heating efficiency on PAHs, traced by ([OI]63um+[OI]145um+[CII]158um) / (PAH+[OI]63um+[OI]145um+[CII]158um), shows a much better match between the observations and the theoretical estimates. The good agreement of the photoelectric heating efficiency on PAHs with a theoretical model indicates the dominant contribution of PAHs to the photoelectric heating. (abridged for arXiv)

Spectrally resolved C II emission in M 33 (HerM33es)

This work presents high spectral resolution observations of the \CII\ line at 158 \micron, one of the major cooling lines of the interstellar medium, taken with the HIFI heterodyne spectrometer on the Herschel satellite. In BCLMP 691, an \HII\ region far north (3.3 kpc) in the disk of M 33, the \CII\ and CO line profiles show similar velocities within $0.5 \kms$, while the \HI\ line velocities are systematically shifted towards lower rotation velocities by $\sim 5\kms$. Observed at the same $12"$ angular resolution, the \CII\ lines are broader than those of CO by about 50% but narrower than the \HI\ lines. The \CII\ line to far-infrared continuum ratio suggests a photoelectric heating efficiency of 1.1%. The data, together with published models indicate a UV field $G_0 \sim 100$ in units of the solar neighborhood value, a gas density $n_H \sim 1000 \cc$, and a gas temperature $T\sim 200$ K. Adopting these values, we estimate the C$^+$ column density to be $N_{C^+} \approx 1.3 \times 10^{17} \cmt$. The \CII\ emission comes predominantly from the warm neutral region between the \HII\ region and the cool molecular cloud behind it. From published abundances, the inferred C$^+$ column corresponds to a hydrogen column density of $N_H \sim 2 \times 10^{21} \cmt$. The CO observations suggest that $N_H = 2 N_{H_2} \sim 3.2 \times 10^{21} \cmt$ and 21cm measurements, also at $12"$ resolution, yield $N_\HI \approx 1.2 \times 10^{21} \cmt$ within the \CII\ velocity range. Thus, some H$_2$ not detected in CO must be present, in agreement with earlier findings based on the SPIRE 250 -- 500 $\mu$m emission.

A STUDY OF HEATING AND COOLING OF THE ISM IN NGC 1097 WITHHERSCHEL-PACS ANDSPITZER-IRS

NGC 1097 is a nearby Seyfert 1 galaxy with a bright circumnuclear starburst ring, a strong large-scale bar and an active nucleus. We present a detailed study of the spatial variation of the far infrared (FIR) [CII]158um and [OI]63um lines and mid-infrared H2 emission lines as tracers of gas cooling, and of the polycyclic aromatic hydrocarbon (PAH) bands as tracers of the photoelectric heating, using Herschel-PACS, and Spitzer-IRS infrared spectral maps. We focus on the nucleus and the ring, and two star forming regions (Enuc N and Enuc S). We estimated a photoelectric gas heating efficiency ([CII]158um+[OI]63um)/PAH in the ring about 50% lower than in Enuc N and S. The average 11.3/7.7um PAH ratio is also lower in the ring, which may suggest a larger fraction of ionized PAHs, but no clear correlation with [CII]158{\mu}m/PAH(5.5 - 14um) is found. PAHs in the ring are responsible for a factor of two more [CII]158um and [OI]63um emission per unit mass than PAHs in the Enuc S. SED modeling indicates that at most 25% of the FIR power in the ring and Enuc S can come from high intensity photodissociation regions (PDRs), in which case G0 ~ 10^2.3 and nH ~ 10^3.5 cm^-3 in the ring. For these values of G0 and nH PDR models cannot reproduce the observed H2 emission. Much of the the H2 emission in the starburst ring could come from warm regions in the diffuse ISM that are heated by turbulent dissipation or shocks.

RESOLVING THE FAR-IR LINE DEFICIT: PHOTOELECTRIC HEATING AND FAR-IR LINE COOLING IN NGC 1097 AND NGC 4559

The physical state of interstellar gas and dust is dependent on the processes which heat and cool this medium. To probe heating and cooling of the ISM over a large range of infrared surface brightness, on sub-kiloparsec scales, we employ line maps of [C \ii] 158 $\mu$m, [O \one] 63 $\mu$m, and [N \ii] 122 $\mu$m in NGC 1097 and NGC 4559, obtained with the PACS spectrometer onboard {\it Herschel}. We matched new observations to existing Spitzer-IRS data that trace the total emission of polycyclic aromatic hydrocarbons (PAHs). We confirm at small scales in these galaxies that the canonical measure of photoelectric heating efficiency, ([C \ii] + [O \one])/TIR, decreases as the far-infrared color, $\nu f_\nu$(70 $\mu$m)/$\nu f_\nu$(100 $\mu$m), increases. In contrast, the ratio of far-infrared (far-IR) cooling to total PAH emission, ([C \ii] + [O \one])/PAH, is a near constant $\sim$6% over a wide range of far-infrared color, 0.5 \textless\ $\nu f_\nu$(70 $\mu$m)/$\nu f_\nu$(100 $\mu$m) $\lesssim$ 0.95. In the warmest regions, where $\nu f_\nu$(70 $\mu$m)/$\nu f_\nu$(100 $\mu$m) $\gtrsim$ 0.95, the ratio ([C \ii] + [O \one])/PAH drops rapidly to 4%. We derived representative values of the local UV radiation density, $G_0$, and the gas density, $n_H$, by comparing our observations to models of photodissociation regions. The ratio $G_0/n_H$, derived from fine-structure lines, is found to correlate with the mean dust-weighted starlight intensity, $<U>$ derived from models of the IR SED. Emission from regions that exhibit a line deficit is characterized by an intense radiation field, indicating that small grains are susceptible to ionization effects. We note that there is a shift in the 7.7 / 11.3 $\mu$m PAH ratio in regions that exhibit a deficit in ([C \ii] + [O \one])/PAH, suggesting that small grains are ionized in these environments.

ASPITZER SPACE TELESCOPEFAR-INFRARED SPECTRAL ATLAS OF COMPACT SOURCES IN THE MAGELLANIC CLOUDS. I. THE LARGE MAGELLANIC CLOUD

[abridged] We present 52-93 micron spectra obtained with Spitzer in the MIPS-SED mode, of a representative sample of luminous compact far-IR sources in the LMC. These include carbon stars, OH/IR AGB stars, post-AGB objects and PNe, RCrB-type star HV2671, OH/IR red supergiants WOHG064 and IRAS05280-6910, B[e] stars IRAS04530-6916, R66 and R126, Wolf-Rayet star Brey3a, Luminous Blue Variable R71, supernova remnant N49, a large number of young stellar objects, compact HII regions and molecular cores, and a background galaxy (z~0.175). We use the spectra to constrain the presence and temperature of cold dust and the excitation conditions and shocks within the neutral and ionized gas, in the circumstellar environments and interfaces with the surrounding ISM. Evolved stars, including LBV R71, lack cold dust except in some cases where we argue that this is swept-up ISM. This leads to an estimate of the duration of the prolific dust-producing phase ("superwind") of several thousand years for both RSGs and massive AGB stars, with a similar fractional mass loss experienced despite the different masses. We tentatively detect line emission from neutral oxygen in the extreme RSG WOHG064, with implications for the wind driving. In N49, the shock between the supernova ejecta and ISM is revealed by its strong [OI] 63-micron emission and possibly water vapour; we estimate that 0.2 Msun of ISM dust was swept up. Some of the compact HII regions display pronounced [OIII] 88-micron emission. The efficiency of photo-electric heating in the interfaces of ionized gas and molecular clouds is estimated at 0.1-0.3%. We confirm earlier indications of a low nitrogen content in the LMC. Evidence for solid state emission features is found in both young and evolved object; some of the YSOs are found to contain crystalline water ice.

A spatially resolved study of photoelectric heating and [C II] cooling in the LMC

<i>Context. <i/>Photoelectric heating is a dominant heating mechanism for many phases of the interstellar medium. We study this mechanism throughout the Large Magellanic Cloud (LMC).<i>Aims. <i/>We aim to quantify the importance of the [C II] cooling line and the photoelectric heating process of various environments in the LMC and to investigate which parameters control the extent of photoelectric heating.<i>Methods. <i/>We use the BICE [C II] map and the Spitzer/SAGE infrared maps. We examine the spatial variations in the efficiency of photoelectric heating: photoelectric heating rate over power absorbed by grains, i.e. the observed [C II] line strength over the integrated infrared emission. We correlate the photoelectric heating efficiency and the emission from various dust constituents and study the variations as a function of H<i>α<i/> emission, dust temperatures, and the total infrared luminosity. The observed variations are interpreted in a theoretical framework. From this we estimate radiation field, gas temperature, and electron density.<i>Results. <i/>We find systematic variations in photoelectric efficiency. The highest efficiencies are found in the diffuse medium, while the lowest coincide with bright star-forming regions (~1.4 times lower). The [C II] line emission constitutes 1.32% of the far infrared luminosity across the whole of the LMC. We find correlations between the [C II] emission and ratios of the mid infrared and far infrared bands, which comprise various dust constituents. The correlations are interpreted in light of the spatial variations of the dust abundance and by the local environmental conditions that affect the dust emission properties. As a function of the total infrared surface brightness, <i>S<i/><sub>TIR<sub/>, the [C II] surface brightness can be described as: = 1.25 [ 10<sup>-3<sup/> erg s<sup>-1<sup/> cm<sup>-2<sup/> sr<sup>-1<sup/>] , for <i>S<i/><sub>TIR<sub/> 3.2 10<sup>-4<sup/> erg s<sup>-1<sup/> cm<sup>-2<sup/> sr<sup>-1<sup/>. We provide a simple model of the photoelectric efficiency as a function of the total infrared luminosity. We find a power-law relation between radiation field and electron density, consistent with other studies. The [C II] emission is well-correlated with the 8 <i>μ<i/>m emission, suggesting that the polycyclic aromatic hydrocarbons play a dominant role in the photoelectric heating process.

Millimeter and Submillimeter Emissions from Galactic and Extragalactic Photodissociation Regions

The galactic and Extragalactic photodissociation regions are primarily heated by photoelectrons ejected from the surface of interstellar dust grains by Far-ultraviolet (FUV) photons. But there is no direct mechanism to measure the photoelectric heating efficiency. To understand the role of dust grains in processing the Interstellar Radiation Field (ISRF) and heating the gas, we compare the intensities ICII, ICO and IFIR for ( 2 P3/2 → 2 P1/2 )&( J = 1 → 0) line emis- sion of CII & CO at 158 μm & 2.6 mm and integrated far- infrared from number of photodissociation regions, HII re- gions, planetary nebulae, reflection nebulae and high latitude translucent clouds (HLCs). It is found that ICII is linearly cor- related with IFIR. In the cold medium where cloud is exposed to weak radiations temperature is low and most of the cool- ing is due to (CII) emissions. As a result the ratio of ICII/IFIR provide indirect method to evaluate the photoelectric heat- ing efficiency. For the neutral cold medium it is evaluated to be ∼0.028. The FUV radiation field G0 are estimated through the model calculation of ICII and ICO for different galactic and photodissociation regions. The intensity of FIR radiation IFIR are well represented as 1.23 × 10 −4 G0(ergs cm −2 s −1 sr −1 ) almost same as estimated for HLCs by Ingalls et al. (2002). Hydrogen density for each source has also been estimated.