Far-infrared Line Emission Research Articles

Constraining the physical properties of gas in high-z galaxies with far-infrared and submillimetre line ratios

Optical emission line diagnostics, which are a common tool for constraining the properties of the interstellar medium (ISM) of galaxies, become progressively inaccessible at higher redshifts for ground-based facilities. Far-infrared (FIR) emission lines, which are redshifted into atmospheric windows that are accessible for ground-based submillimetre facilities, could provide ISM diagnostics alternative to optical emission lines. We investigated FIR line ratios involving CII OIII OIII NII and NIII using synthetic emission lines applied to a high-resolution gas $= 883.4 M$_ odot $) cosmological zoom-in simulation, including radiative transfer post-processing with the code at z = 6.5. We find that the CII NII 122 ratio is sensitive to the temperature and density of photodissociation regions. It might therefore be a useful tool for tracing the properties of this gas phase in galaxies. We also find that NII NIII is a good tracer of the temperature and that OIII OIII 88 is a good tracer of the gas density of HII regions. Emission line ratios containing the OIII line are sensitive to high-velocity outflowing gas.

Atacama Large Aperture Submillimeter Telescope (AtLAST) science: The hidden circumgalactic medium.

Our knowledge of galaxy formation and evolution has incredibly progressed through multi-wavelength observational constraints of the interstellar medium (ISM) of galaxies at all cosmic epochs. However, little is known about the physical properties of the more diffuse and lower surface brightness reservoir of gas and dust that extends beyond ISM scales and fills dark matter haloes of galaxies up to their virial radii, the circumgalactic medium (CGM). New theoretical studies increasingly stress the relevance of the latter for understanding the feedback and feeding mechanisms that shape galaxies across cosmic times, whose cumulative effects leave clear imprints into the CGM. Recent studies are showing that a - so far unconstrained - fraction of the CGM mass may reside in the cold ( T < 10 4 K) molecular and atomic phase, especially in high-redshift dense environments. These gas phases, together with the warmer ionised phase, can be studied in galaxies from z ∼ 0 to z ∼ 10 through bright far-infrared and sub-millimeter emission lines such as [C ii] 158 µm, [O iii] 88 µm, [C I] 609 µm, [C i] 370 µm, and the rotational transitions of CO. Imaging such hidden cold CGM can lead to a breakthrough in galaxy evolution studies but requires a new facility with the specifications of the proposed Atacama Large Aperture Submillimeter Telescope (AtLAST). In this paper, we use theoretical and empirical arguments to motivate future ambitious CGM observations with AtLAST and describe the technical requirements needed for the telescope and its instrumentation to perform such science.

A benchmark for extreme conditions of the multiphase interstellar medium in the most luminous hot dust-obscured galaxy at z = 4.6

WISE J224607.6–052634.9 (W2246–0526) is a hot dust-obscured galaxy at z = 4.601, and the most luminous obscured quasar known to date. W2246–0526 harbors a heavily obscured supermassive black hole that is most likely accreting above the Eddington limit. We present observations with the Atacama Large Millimeter/submillimeter Array (ALMA) in seven bands, including band 10, of the brightest far-infrared (FIR) fine-structure emission lines of this galaxy: [OI]63 μm, [OIII]88 μm, [NII]122 μm, [OI]145 μm, [CII]158 μm, [NII]205 μm, [CI]370 μm, and [CI]609 μm. A comparison of the data to a large grid of CLOUDY radiative transfer models reveals that a high hydrogen density (nH ∼ 3 × 103 cm−3) and extinction (AV ∼ 300 mag), together with extreme ionization (log(U) = − 0.5) and a high X-ray to UV ratio (αox ≥ −0.8) are required to reproduce the observed nuclear line ratios. The values of αox and U are among the largest found in the literature and imply the existence of an X-ray-dominated region (XDR). In fact, this component explains the a priori very surprising non-detection of the [OIII]88 μm emission line, which is actually suppressed, instead of boosted, in XDR environments. Interestingly, the best-fitted model implies higher X-ray emission and lower CO content than what is detected observationally, suggesting the presence of a molecular gas component that should be further obscuring the X-ray emission over larger spatial scales than the central region that is being modeled. These results highlight the need for multiline infrared observations to characterize the multiphase gas in high redshift quasars and, in particular, W2246–0526 serves as an extreme benchmark for comparisons of interstellar medium conditions with other quasar populations at cosmic noon and beyond.

Diagnosing the interstellar medium of galaxies with far-infrared emission lines

Context. Gas cooling processes in the interstellar medium (ISM) are key to understanding how star formation occurs in galaxies. Far-infrared (FIR) fine-structure emission lines can be used to infer gas conditions and trace different phases of the ISM. Aims. We model eight of the most important FIR emission lines and explore their variation with star formation rate (SFR) out to z = 6 using cosmological hydrodynamical simulations. In addition, we study how different physical parameters, such as the interstellar radiation field (ISRF) and metallicity, impact the FIR lines and line ratios. Methods. We implemented a physically motivated multi-phase model of the ISM by post-processing the EAGLE cosmological simulation and using CLOUDY look-up tables for line emissivities. In this model we included four phases of the ISM: dense molecular gas, neutral atomic gas, diffuse ionised gas (DIG), and H II regions. Results. Our model shows reasonable agreement (to ∼0.5 dex) with the observed line luminosity–SFR relations up to z = 6 in the FIR lines analysed. For ease of comparison, we also provide linear fits to our model results. Our predictions also agree reasonably well with observations in diagnostic diagrams involving various FIR line ratios. Conclusions. We find that [C II] is the best SFR tracer of the FIR lines even though it arises from multiple ISM phases, while [O III] and [N II] can be used to understand the DIG–H II balance in the ionised gas. In addition, line ratios such as [C II]/[O III] and [N II]/[O I] are useful for deriving parameters such as ISRF, metallicity, and specific SFR. These results can help interpret the observations of the FIR lines from the local Universe to high redshifts.

Far-infrared line emission from the outer Galaxy cluster Gy 3–7 with SOFIA/FIFI-LS: Physical conditions and UV fields

Context. Far-infrared (FIR) line emission provides key information about the gas cooling and heating due to shocks and UV radiation associated with the early stages of star formation. Gas cooling via FIR lines might, however, depend on metallicity. Aims. We aim to quantify the FIR line emission and determine the spatial distribution of the CO rotational temperature, ultraviolet (UV) radiation field, and H2 number density toward the embedded cluster Gy 3–7 in the CMa–l224 star-forming region, whose metallicity is expected to be intermediate between that of the Large Magellanic Cloud and the Solar neighborhood. By comparing the total luminosities of CO and [OI] toward Gy 3–7 with values found for low- and high-mass protostars extending over a broad range of metallicities, we also aim to identify the possible effects of metallicity on the FIR line cooling within our Galaxy. Methods. We studied SOFIA/FIFI-LS spectra of Gy 3–7, covering several CO transitions from J = 14–13 to 31-30, the OH doublet at 79 μm, the [OI] 63.2 and 145.5 μm, and the [CII] 158 μm lines. The field of view covers a 2′ × 1′ region with a resolution of ~7″–18″. Results. The spatial extent of CO high-J (Jup ≥14) emission resembles that of the elongated 160 μm continuum emission detected with Herschel, but its peaks are offset from the positions of the dense cores. The [OI] lines at 63.2 μm and 145.5 μm follow a similar pattern, but their peaks are found closer to the positions of the cores. The CO transitions from J = 14–13 to J = 16–15 are detected throughout the cluster and show a median rotational temperature of 170 ± 30 K on Boltzmann diagrams. Comparisons to other protostars observed with Berschel show a good agreement with intermediate-mass sources in the inner Galaxy. Assuming an origin of the [OI] and high-J CO emission in UV-irradiated C–shocks, we obtained pre-shock H2 number densities of 104–105 cm−3 and UV radiation field strengths of 0.1–10 Habing fields (G0). Conclusions. Far-IR line observations reveal ongoing star formation in Gy 3–7, dominated by intermediate-mass Class 0/I young stellar objects. The ratio of molecular-to-atomic far-IR line emission shows a decreasing trend with bolometric luminosities of the protostars. However, it does not indicate that the low-metallicity has an impact on the line cooling in Gy 3–7.

Fine-structure transitions of Si and S induced by collisions with atomic hydrogen

ABSTRACT Using a quantum-mechanical close-coupling method, we calculate cross-sections for fine-structure excitation and relaxation of Si and S atoms in collisions with atomic hydrogen. Rate coefficients are calculated over a range of temperatures for astrophysical applications. We determine the temperature-dependent critical densities for the relaxation of Si and S in collisions with H and compare these to the critical densities for collisions with electrons. The present calculations should be useful in modelling environments exhibiting the [S i] 25 μm and [S i] 57 μm far-infrared emission lines or where cooling of S and Si by collisions with H is of interest.

ALMA Observations of CO Emission from Luminous Lyman-break Galaxies at z = 6.0293–6.2037

We present our new Atacama Large Millimeter/submillimeter Array (ALMA) observations targeting CO(6–5) emission from three luminous Lyman-break galaxies (LBGs) at z spec = 6.0293–6.2037 found in the Subaru/Hyper Suprime-Cam survey, whose [O iii] 88 μm and [C ii] 158 μm emissions have been detected with ALMA. We find a marginal detection of the CO(6–5) line from one of our LBGs, J0235–0532, at the ≃4σ significance level and obtain upper limits for the other two LBGs, J1211–0118 and J0217–0208. Our z = 6 luminous LBGs are consistent with the previously found correlation between the CO luminosity and the infrared luminosity. The unique ensemble of the multiple far-infrared emission lines and underlying continuum fed to a photodissociation region model reveals that J0235–0532 has a relatively high density of hydrogen nuclei n H that is comparable to those of low-z (U)LIRGs, quasars, and Galactic star-forming regions with high n H values, while the other two LBGs have lower n H consistent with local star-forming galaxies. By carefully taking account of various uncertainties, we obtain constraints on total gas mass and gas surface density from their CO luminosity measurements. We find that J0235–0532 is located below the Kennicutt–Schmidt (KS) relation, comparable to the z = 5.7 LBG, HZ10, previously detected with CO(2–1). Combined with previous results for dusty starbursts at similar redshifts, the KS relation at z = 5–6 is on average consistent with the local one.

Investigating the [C ii]-to-H i Conversion Factor and the H i Gas Budget of Galaxies at z ≈ 6 with Hydrodynamic Simulations

One of the most fundamental baryonic matter components of galaxies is the neutral atomic hydrogen (H i). At low redshifts, this component can be traced directly through the 21 cm transition, but to infer the H i gas content of the most distant galaxies, a viable tracer is needed. We here investigate the fidelity of the fine-structure transition of the (2 P 3/2 − 2 P 1/3) transition of singly ionized carbon C ii at 158 μm as a proxy for H i in a set simulated galaxies at z ≈ 6, following the work by Heintz et al. We select 11,125 star-forming galaxies from the simba simulations, with far-infrared line emissions postprocessed and modeled within the Sigame framework. We find a strong connection between C ii and H i, with the relation between this C ii-to-H i relation (β [C II]) being anticorrelated with the gas-phase metallicity of the simulated galaxies. We further use these simulations to make predictions for the total baryonic matter content of galaxies at z ≈ 6, and specifically the H i gas mass fraction. We find mean values of M H I/M ⋆ = 1.4 and M H I/M bar,tot = 0.45. These results provide strong evidence for H i being the dominant baryonic matter component by mass in galaxies at z ≈ 6.

Sígame v3: Gas Fragmentation in Postprocessing of Cosmological Simulations for More Accurate Infrared Line Emission Modeling

Abstract We present an update to the framework called Simulator of Galaxy Millimeter/submillimeter Emission (sígame). sígame derives line emission in the far-infrared (FIR) for galaxies in particle-based cosmological hydrodynamics simulations by applying radiative transfer and physics recipes via a postprocessing step after completion of the simulation. In this version, a new technique is developed to model higher gas densities by parameterizing the probability distribution function (PDF) of the gas density in higher-resolution simulations run with the pseudo-Lagrangian, Voronoi mesh code arepo. The parameterized PDFs are used as a look-up table, and reach higher densities than in previous work. sígame v3 is tested on redshift z = 0 galaxies drawn from the simba cosmological simulation for eight FIR emission lines tracing vastly different phases of the interstellar medium. This version of sígame includes dust radiative transfer with Skirt and high-resolution photoionization models with Cloudy, the latter sampled according to the density PDF of the arepo simulations to augment the densities in the cosmological simulation. The quartile distributions of the predicted line luminosities overlap with the observed range for nearby galaxies of similar star formation rate (SFR) for all but two emission lines: [O i]63 and CO(3–2), which are overestimated by median factors of 1.3 and 1.0 dex, respectively, compared to the observed line–SFR relation of mixed-type galaxies. We attribute the remaining disagreement with observations to the lack of precise attenuation of the interstellar light on sub-grid scales (≲200 pc) and differences in sample selection.

The Impact of Powerful Jets on the Far-infrared Emission of an Extreme Radio Quasar at z ∼ 6

Abstract The interactions between radio jets and the interstellar medium play a defining role for the coevolution of central supermassive black holes and their host galaxies, but observational constraints on these feedback processes are still very limited at redshifts z &gt; 2. We investigate the radio-loud quasar PSO J352.4034–15.3373 at z ∼ 6 at the edge of the Epoch of Reionization. This quasar is among the most powerful radio emitters and the first one with direct evidence of extended radio jets (∼1.6 kpc) at these high redshifts. We analyze NOrthern Extended Millimeter Array and Atacama Large Millimeter/submillimeter Array millimeter data targeting the CO (6–5) and [C ii] far-infrared (FIR) emission lines, respectively, and the underlying continuum. The broad 440 ± 80 km s−1 and marginally resolved [C ii] emission line yields a systemic redshift of z = 5.832 ± 0.001. Additionally, we report a strong 215 MHz radio continuum detection, 88 ± 7 mJy, using the Giant Metrewave Radio Telescope. This measurement significantly improves the constraints at the low-frequency end of the spectral energy distribution of this quasar. In contrast to what is typically observed in high-redshift radio-quiet quasars, we show that cold dust emission alone cannot reproduce the millimeter continuum measurements. This is evidence that the strong synchrotron emission from the quasar contributes substantially to the emission even at millimeter (FIR in the rest-frame) wavelengths. This quasar is an ideal system to probe the effects of radio jets during the formation of a massive galaxy within the first gigayear of the universe.

C II] 158 μm line emission from Orion A I. A template for extragalactic studies?

Context. The [C II] 158 μm fine-structure line is one of the dominant coolants of the neutral interstellar medium. It is hence one of the brightest far-infrared (FIR) emission lines and can be observed not only in star-forming regions throughout the Galaxy, but also in the diffuse interstellar medium and in distant galaxies. [C II] line emission has been suggested to be a powerful tracer of star formation. Aims. We aim to understand the origin of [C II] emission and its relation to other tracers of interstellar gas and dust. This includes a study of the heating efficiency of interstellar gas as traced by the [C II] line to test models of gas heating. Methods. We made use of a one-square-degree map of velocity-resolved [C II] line emission toward the Orion Nebula complex, including M 43 and NGC 1977. We employed Herschel FIR photometric images to determine dust properties. Moreover, we compared with Hα emission from the ionized gas, Spitzer mid-infrared photometry to trace hot dust and large polycyclic aromatic hydrocarbons (PAHs), 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 FIR emission from warm dust. However, the [C II] intensity depends less than linearly on the 8 μm and FIR intensity, while 8 μm and FIR intensities are approximately linearly correlated. The correlation between [C II] and CO(2–1) does not show a clear trend and is affected by the detailed geometry of the region. We find particularly low [C II]-over-FIR intensity ratios toward large columns of (warm and cold) dust, which suggest the interpretation of the “[C II] deficit” in terms of a “FIR excess”. Conclusions. In terms of the [C II] deficit, we find clear evidence in our data for the importance of [O I] 63 μm emission in the photodissociation regions (PDRs) associated with the Huygens region. A smaller contribution is made by a decreased heating efficiency in regions of high UV irradiation. FIR emission from deeply embedded protostars leads to palpably deficient [C II]/FIR intensity ratios. The [C II] directly associated with the M 42, M 43, and NGC 1977 regions underestimates the star formation rate derived from extragalactic scaling relations. We ascribe this to the importance of [C II] emission from low surface brightness PDR surfaces of molecular clouds which are not included in our survey. Future studies of more active regions of massive star formation will be instrumental in validating the general applicability of these conclusions.

A low [CII]/[NII] ratio in the center of a massive galaxy at z = 3.7: Evidence for a transition to quiescence at high redshift? (Corrigendum)

Understanding the process of quenching is one of the major open questions in galaxy evolution, and crucial insights may be obtained by studying quenched galaxies at high redshifts, at epochs when the Universe and the galaxies were younger and simpler to model. However, establishing the degree of quiescence in high redshift galaxies is a challenging task. One notable example is Hyde, a recently discovered galaxy at z=3.709. As compact (r~0.5 kpc) and massive (M*~1e11 Msun) as its quenched neighbor Jekyll, it is also extremely obscured yet only moderately luminous in the sub-millimeter. Panchromatic modeling suggested it could be the first galaxy found in transition to quenching at z>3, however the data were also consistent with a broad range of star-formation activity, including moderate SFR in the lower scatter of the galaxy main-sequence (MS). Here, we describe ALMA observations of the [CII] 157um and [NII] 205um far-infrared emission lines. The [CII] emission within the half-light radius is dominated by ionized gas, while the outskirts are dominated by PDRs or neutral gas. This suggests that the ionization in the center is not primarily powered by on-going star formation, and could come instead from remnant stellar populations formed in an older burst, or from a moderate AGN. Accounting for this information in the multi-wavelength modeling provides a tighter constraint on the star formation rate of SFR=$50^{+24}_{-18}$ Msun/yr. This rules out fully quenched solutions, and favors SFRs more than factor of two lower than expected for a galaxy on the MS, confirming the nature of Hyde as a transition galaxy. Theses results suggest that quenching happens from inside-out, and starts before the galaxy expels or consumes all its gas reservoirs. Similar observations of a larger sample would determine whether this is an isolated case or the norm for quenching at high-redshift. [abriged]

The dust mass in Cassiopeia A from infrared and optical line flux differences

ABSTRACT The large quantities of dust that have been found in a number of high-redshift galaxies have led to suggestions that core-collapse supernovae (CCSNe) are the main sources of their dust and have motivated the measurement of the dust masses formed by local CCSNe. For Cassiopeia A (Cas A), an oxygen-rich remnant of a Type IIb CCSN, a dust mass of 0.6–1.1 M⊙ has already been determined by two different methods, namely (a) from its far-infrared spectral energy distribution and (b) from analysis of the red–blue emission line asymmetries in its integrated optical spectrum. We present a third, independent, method for determining the mass of dust contained within Cas A. This compares the relative fluxes measured in similar apertures from [O iii] far-infrared and visual-region emission lines, taking into account foreground dust extinction, in order to determine internal dust optical depths, from which corresponding dust masses can be obtained. Using this method, we determine a dust mass within Cas A of at least 0.99$^{+0.10}_{-0.09}$ M⊙.

The case for thermalization as a contributor to the [C ii] deficit

ABSTRACT The [C ii] deficit, which describes the observed decrease in the ratio of [C ii] 158 μm emission to continuum infrared emission in galaxies with high star formation surface densities, places a significant challenge to the interpretation of [C ii] detections from across the observable universe. In an attempt to further decode the cause of the [C ii] deficit, the [C ii] and dust continuum emission from 18 Local Volume galaxies has been split based on conditions within the interstellar medium where it originated. This is completed using the Key Insights in Nearby Galaxies: a Far-Infrared Survey with Herschel (KINGFISH) and Beyond the Peak (BtP) surveys and the wide-range of wavelength information, from UV to far-infrared emission lines, available for a selection of star-forming regions within these samples. By comparing these subdivided [C ii] emissions to isolated infrared emission and other properties, we find that the thermalization (collisional de-excitation) of the [C ii] line in H ii regions plays a significant role in the deficit observed in our sample.

A low [CII]/[NII] ratio in the center of a massive galaxy at z = 3.7: Evidence for a transition to quiescence at high redshift?

Understanding the process of quenching is one of the major open questions in galaxy evolution and crucial insights may be obtained by studying quenched galaxies at high redshifts at epochs when the Universe and the galaxies were younger and simpler to model. However, establishing the degree of quiescence in high-redshift galaxies is a challenging task. One notable example is Hyde, a recently discovered galaxy at zspec = 3.709. Equally as compact (r1/2 ∼ 0.5 kpc) and massive (M* ∼ 1011 M⊙) as its quenched neighbor Jekyll, it is also extremely obscured yet only moderately luminous in the sub-millimeter. Panchromatic modeling has suggested it could be the first galaxy found in transition to quenching at z &gt; 3, however, the data are also consistent with a broad range of star-formation activity, from fully quenched to moderate star-formation rates (SFR) in the lower scatter of the galaxy main-sequence. Here, we describe Atacama Large Millimeter Array observations of the [C II] 157 μm and [N II] 205 μm far-infrared emission lines. The [C II] emission within the half-light radius is dominated by ionized gas, while the outskirts are dominated by photo-dissociation regions or neutral gas. This suggests that the ionization in the center is not primarily powered by ongoing star formation, and is instead coming from remnant stellar populations formed in an older burst or from a moderate active galactic nucleus . Accounting for this information in the multi-wavelength modeling provides a tighter constraint on the star formation rate of SFR = 50−18+24 M⊙ yr−1 yr−1. This rules out fully quenched solutions and favors SFRs more than factor of two lower than expected for a main-sequence galaxy, confirming the nature of Hyde as a transition galaxy. These results suggest that quenching happens from inside-out and starts before the galaxy expels or consumes all its gas reservoirs. Similar observations of a sample of massive and obscured galaxies would determine whether this is an isolated case or the norm for quenching at high redshift.

Diagnosing the interstellar medium of galaxies with far-infrared emission lines

Context. Atomic fine structure lines have been detected in the local Universe and at high redshifts over the past decades. The [C II] emission line at 158 μm is an important observable as it provides constraints on the interstellar medium (ISM) cooling processes. Aims. We develop a physically motivated framework to simulate the production of far-infrared line emission from galaxies in a cosmological context. This first paper sets out our methodology and describes its first application: simulating the [C II] 158 μm line emission in the local Universe. Methods. We combine the output from EAGLE cosmological hydrodynamical simulations with a multi-phase model of the ISM. Gas particles are divided into three phases: dense molecular gas, neutral atomic gas, and diffuse ionised gas (DIG). We estimate the [C II] line emission from the three phases using a set of CLOUDY cooling tables. Results. Our results agree with previous findings regarding the contribution of these three ISM phases to the [C II] emission. Our model shows good agreement with the observed L[C II–star formation rate (SFR) relation in the local Universe within 0.4 dex scatter. Conclusions. The fractional contribution to the [C II] line from different ISM phases depends on the total SFR and metallicity. The neutral gas phase dominates the [C II] emission in galaxies with SFR ∼ 0.01–1 M⊙ yr−1, but the ionised phase dominates at lower SFRs. Galaxies above solar metallicity exhibit lower L[C II/SFR ratios for the neutral phase. In comparison, the L[C II/SFR ratio in the DIG is stable when metallicity varies. We suggest that the reduced size of the neutral clouds, caused by increased SFRs, is the likely cause for the L[C II deficit at high infrared luminosities, although EAGLE simulations do not reach these luminosities at z = 0.

The Lyman Alpha Reference Sample

Context. Lyman-α (Lyα) is the brightest emission line in star-forming galaxies. However, its interpretation in terms of physical properties is hampered by the resonant nature of Lyα photons. In order to remedy this complicated situation, the Lyman Alpha Reference Sample (LARS) was defined, enabling the study of Lyα production and escape mechanisms in 14 local star-forming galaxies. Aims. With this paper, we complement our efforts and study the global dust and (molecular) gas content as well as the properties of gas associated with photon-dominated regions. We aim to characterize the interstellar medium of LARS galaxies, allowing us to relate these newly derived properties to quantities relevant for Lyα escape. Methods. We observed LARS galaxies with Herschel, SOFIA, the IRAM 30m telescope, and APEX, targeting far-infrared (FIR) continuum and emission lines of [C II]158 μm, [O I]63 μm, [O III]88 μm, and low-J CO lines. Using Bayesian methods we derived dust model parameters and estimated the total gas masses for all LARS galaxies, taking into account a metallicity-dependent gas-to-dust ratio. Star formation rates were estimated from FIR, [C II]158 μm, and [O I]63 μm luminosities. Results. LARS covers a wide dynamic range in the derived properties, with FIR-based star formation rates from ∼0.5−100 M⊙ yr−1, gas fractions between ∼15−80%, and gas depletion times ranging from a few hundred megayears up to more than ten gigayears. The distribution of LARS galaxies in the Σgas versus ΣSFR (Kennicutt–Schmidt plane) is thus quite heterogeneous. However, we find that LARS galaxies with the longest gas depletion times, that is, relatively high gas surface densities (Σgas) and low star formation rate densities (ΣSFR), have by far the highest Lyα escape fraction. A strong approximately linear relation is found between the Lyα escape fraction and the total gas (HI+H2) depletion time. We argue that the Lyα escape in those galaxies is driven by turbulence in the star-forming gas that shifts the Lyα photons out of resonance close to the places where they originate. We further report on an extreme [C II]158 μm excess in LARS 5, corresponding to ∼14 ± 3% of the FIR luminosity, which probably is the most extreme [C II]-to-FIR ratio observed in a galaxy (without active nucleus) to date.

FIFI-LS Observations of the Circumnuclear Ring— Probing the High-density Phase of the PDR

Abstract We investigate the nature of dense gas in the 3–10 pc circumnuclear ring (CNR) in the galactic center of the Milky Way, which is a structure that may be dynamically connecting the supermassive black hole Sgr A* with the central molecular zone at the 100 pc scale, and is the closest reservoir of molecular gas to the massive stars located within the central cluster. In the first of several papers addressing open issues with the CNR, we use far-infrared (FIR) diagnostic emission lines to probe the hot and dense phase of the photodissociation region (PDR) exposed to the radiation field of the central population of massive stars. We use the Far Infrared Field-Imaging Line Spectrometer (FIFI-LS) instrument on board the Stratospheric Observatory For Infrared Astronomy airborne observatory to obtain spatially resolved maps of FIR emission lines of the region with an angular resolution approximately 4 times higher than previous published data. We complement our data with archival continuum images at 19.7, 31.5 and 37.1 μm obtained with FORCAST and 70, 100 and 160 μm archival continuum images from PACS. We use the FIFI-LS emission line flux maps from ionized ([C ii] 157.7 μm), atomic ([O i] 63.2 μm, [O i] 145.5 μm), and molecular (CO J = 14–13 186.0 μm) species for a comparison with model predictions for PDRs. We present a method that dissects emission from the low and from the high excitation phase of the PDR and that also accounts for, e.g., absorption especially in the [O i] 63.2 μm transition. We present spatially resolved maps of dust temperature, atomic hydrogen column density, and FIR flux. The derived atomic hydrogen column density map is aligned with the galactic plane and extends spatially beyond previous near-infrared and radio based A v determinations. The atomic hydrogen column densities range from 1022.5 to 1023.1 cm−2 resulting in a total enclosed mass of the order of 103.5 M ⊙. We derive a [O i] 63.2 μm absorption map that is aligned with the galactic plane with no or little absorption in the northern lobe of the CNR but moderate absorption in the southern lobe of the CNR, which is consistent with the picture where the illuminated front surfaces of gas clouds in the northern lobe are directly visible to us, while in the southern lobe the illuminated surfaces are hidden by the clouds within the lobe itself. Local gas densities in the CNR are generally below the Roche limit.

Census of and Lyα, [Oiii]5007, Hα, and [Ciii]158 μm line emission wiht ∼1000 galaxies atz= 4.9 – 7.0 revealed with Subaru/HSC, Spitzer, and ALMA

AbstractWe investigate rest-frame UV to far-infrared emission lines and SEDs from 1124 galaxies atz= 4.9 – 7.0. Our sample is composed of 1092 Lyα emitters (LAEs) atz= 4.9–7.0 identified by Subaru/Hyper Suprime-Cam (HSC) narrowband surveys and 34 galaxies atz= 5.148–7.508 with deep [Cii]158μm ALMA data. The SEDs clearly show flux excesses in the Spitzer/IRAC 3.6 and 4.5μm bands, suggesting strong rest-frame optical emission lines of [Oiii] and/or Hα. We model the galaxy SEDs with a flexible code combining stellar population and photoionization models (BEAGLE; Chevallard &amp; Charlot 2016), and investigate relations between the emission lines of Lyα, [Oiii], Hα, and [CII]. We find 1) a positive correlation between the rest-frame Hα equivalent width (EW) and the Lyα, EW,${\EW {^0_Ly\alpha}} $, 2) an interesting turn-over trend that the [Oiii]/Hα flux ratio increases in${\EW {^0_Ly\alpha}} $≃ 0–30 Å, and then decreases out to${\EW {^0_Ly\alpha}} $≃ 130 Å, and 3) a &gt; 99% anti-correlation between a [Cii] luminosity to star-formation rate ratioL[Cii]/SFR) and${\EW {^0_Ly\alpha}} $. Modeling with BEAGLE also suggests that a simple anticorrelation between${\EW {^0_Ly\alpha}} $and metallicity explains self-consistently all of the relations of Lyα, Hα, [Oiii]/Hα, and [Cii] in our study, indicative of detections of very metal-poor (∼0.03Z⊙) galaxies with${\EW {^0_Ly\alpha}} $∼200 Å.

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.