Far-infrared Fine-structure Lines Research Articles

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.

Tracing the Layers of Photodissociated Gas in the Trifid Nebula

Photodissociated gas bears the signature of the dynamical evolution of the ambient interstellar medium impacted by the mechanical and radiative feedback from an expanding H ii region. Here we present an analysis of the kinematics of the young Trifid Nebula, based on velocity-resolved observations of the far-infrared fine structure lines of [C ii] at 158 μm and [O i] at 63 μm. The distribution of the photodissociated regions (PDRs) surrounding the nebula is consistent with a shell-like structure created by the H ii region expanding at a velocity of 5 km s−1. Comparison of ratios of [C ii] and [O i]63 μm intensities for identical velocity components with PDR models indicate a density of 104 cm−3. The redshifted and blueshifted PDR shells with a combined mass of 516 M ⊙ have a kinetic energy of ∼1047 erg. This is consistent with the thermal energy of the H ii region as well as with the energy deposited by the stellar wind luminosity from HD 169442A, an O7 V star, over the 0.5 Myr lifetime of the star. The observed momentum of the PDR shell is lower than what theoretical calculations predict for the radial momentum due to the shell being swept up by an expanding H ii region, which suggests that significant mass loss has occurred in M20 due to the dispersal of the surrounding gas by the advancing ionization front.

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.

The PhotoDissociation Region Toolbox: Software and Models for Astrophysical Analysis

The PhotoDissociation Region Toolbox provides comprehensive, easy-to-use, public software tools and models that enable an understanding of the interaction of the light of young, luminous, massive stars with the gas and dust in the Milky Way and in other galaxies. It consists of an open-source Python toolkit and photodissociation region (PDR) models for analysis of infrared and millimeter/submillimeter line and continuum observations obtained by ground-based and suborbital telescopes, and astrophysics space missions. PDRs include all of the neutral gas in the interstellar medium where far-ultraviolet photons dominate the chemistry and/or heating. In regions of massive star formation, PDRs are created at the boundaries between the H ii regions and neutral molecular cloud, as photons with energies 6 eV < h ν < 13.6 eV photodissociate molecules and photoionize metals. The gas is heated by photoelectrons from small grains and large molecules and cools mostly through far-infrared (FIR) fine-structure lines like [O i] and [C ii]. The models are created from state-of-the art PDR codes that include molecular freeze-out; recent collision, chemical, and photorates; new chemical pathways, such as oxygen chemistry; and allow for both clumpy and uniform media. The models predict the emergent intensities of many spectral lines and FIR continuum. The tools find the best-fit models to the observations and provide insight into the physical conditions and chemical makeup of the gas and dust. The PDR Toolbox enables novel analysis of data from telescopes such as the Infrared Space Observatory, Spitzer, Herschel, the Stratospheric Terahertz Observatory, the Stratospheric Observatory for Infrared Astronomy, the Submillimeter Wave Astronomy Satellite, the Atacama Pathfinder Experiment, the Atacama Large Millimeter/submillimeter Array, and the JWST.

Direct Far-infrared Metal Abundances (FIRA). I. M101

Abstract Accurately determining gas-phase metal abundances within galaxies is critical as metals strongly affect the physics of the interstellar medium. To date, the vast majority of widely used gas-phase abundance indicators rely on emission from bright optical lines, whose emissivities are highly sensitive to the electron temperature. Alternatively, direct-abundance methods exist that measure the temperature of the emitting gas directly, though these methods usually require challenging observations of highly excited auroral lines. Low-lying far-infrared (FIR) fine structure lines are largely insensitive to electron temperature and thus provide an attractive alternative to optically derived abundances. Here, we introduce the far-infrared abundance (FIRA) project, which employs these FIR transitions, together with both radio free–free emission and hydrogen recombination lines, to derive direct, absolute gas-phase oxygen abundances. Our first target is M101, a nearby spiral galaxy with a relatively steep abundance gradient. Our results are consistent with the O++ electron temperatures and absolute oxygen abundances derived using optical direct-abundance methods by the CHemical Abundance Of Spirals (CHAOS) program, with a small difference (∼1.5σ) in the radial abundance gradients derived by the FIR/free–free-normalized versus CHAOS/direct-abundance techniques. This initial result demonstrates the validity of the FIRA methodology—with the promise of determining absolute metal abundances within dusty star-forming galaxies, both locally and at high redshift.

Ionized Carbon around IRC+10216

Abstract Asymptotic giant branch stars create a rich inventory of molecules in their envelopes as they lose mass during later stages of their evolution. These molecules cannot survive the conditions in interstellar space, where they are exposed to ultraviolet photons of the interstellar radiation field. As a result, daughter molecules are the ones injected into space, and a halo of those molecules is predicted to exist around cool evolved stars. The most abundant molecule in the envelopes other than H2 is CO, which dissociates into C that is rapidly ionized into C+ in a halo around the star that is optically thin to the interstellar radiation field. We develop the specific predictions of the ionized carbon halo size and column density for the well-studied, nearby star IRC+10216. We compare those models to observations of the [C ii] 157.7 μm far-infrared fine structure line using the Stratospheric Observatory for Infrared Astronomy and Herschel. The combination of bright emission toward the star and upper limits to extended [C ii] is inconsistent with any standard model. The presence of [C ii] toward the star requires some dissociation and ionization in the inner part of the outflow, possibly due to a hot companion star. The lack of extended [C ii] emission requires that daughter products from CO photodissociation in the outer envelope remain cold. The [C ii] profile toward the star is asymmetric, with the blueshifted absorption due to the cold outer envelope.

Galaxy Evolution Probe

The Galaxy Evolution Probe (GEP) is a concept for a mid- and far-infrared space observatory to measure key properties of large samples of galaxies with large and unbiased surveys. GEP will attempt to achieve zodiacal light and Galactic dust emission photon background-limited observations by utilizing a 6 Kelvin, 2.0 meter primary mirror and sensitive arrays of kinetic inductance detectors. It will have two instrument modules: a 10 - 400 micron hyperspectral imager with spectral resolution R = 8 (GEP-I) and a 24 - 193 micron, R = 200 grating spectrometer (GEP-S). GEP-I surveys will identify star-forming galaxies via their thermal dust emission and simultaneously measure redshifts using polycyclic aromatic hydrocarbon emission lines. Galaxy luminosities derived from star formation and nuclear supermassive black hole accretion will be measured for each source, enabling the cosmic star formation history to be measured to much greater precision than previously possible. Using optically thin far-infrared fine-structure lines, surveys with GEP-S will measure the growth of metallicity in the hearts of galaxies over cosmic time and extraplanar gas will be mapped in spiral galaxies in the local universe to investigate feedback processes. The science case and mission architecture designed to meet the science requirements are described, and the kinetic inductance detector and readout electronics state of the art and needed developments are described. This paper supersedes the GEP concept study report cited in it by providing new content, including: a summary of recent mid-infrared KID development, a discussion of microlens array fabrication for mid-infrared KIDs, and additional context for galaxy surveys. The reader interested in more technical details may want to consult the concept study report.

Observation and calibration strategies for large-scale multi-beam velocity-resolved mapping of the [CII] emission in the Orion molecular cloud

Context.The [CII] 158 μm far-infrared fine-structure line is one of the dominant cooling lines of the star-forming interstellar medium. Hence [CII] emission originates in and thus can be used to trace a range of ISM processes. Velocity-resolved large-scale mapping of [CII] in star-forming regions provides a unique perspective of the kinematics of these regions and their interactions with the exciting source of radiation.Aims.We explore the scientific applications of large-scale mapping of velocity-resolved [CII] observations. With the [CII] observations, we investigate the effect of stellar feedback on the ISM. We present the details of observation, calibration, and data reduction using a heterodyne array receiver mounted on an airborne observatory.Methods.A 1.15 square degree velocity-resolved map of the Orion molecular cloud centred on the bar region was observed using the German REceiver for Astronomy at Terahertz Frequencies (upGREAT) heterodyne receiver flying on board the Stratospheric Observatory for Infrared Astronomy. The data were acquired using the 14 pixels of the German REceiver for Astronomy at Terahertz Frequencies that were observed in an on-the-fly mapping mode. 2.4 million spectra were taken in total. These spectra were gridded into a three-dimensional cube with a spatial resolution of 14.1 arcseconds and a spectral resolution of 0.3 km s−1.Results.A square-degree [CII] map with a spectral resolution of 0.3 km s−1is presented. The scientific potential of this data is summarized with discussion of mechanical and radiative stellar feedback, filament tracing using [CII], [CII] opacity effects, [CII] and carbon recombination lines, and [CII] interaction with the large molecular cloud. The data quality and calibration is discussed in detail, and new techniques are presented to mitigate the effects of unavoidable instrument deficiencies (e.g. baseline stability) and thus to improve the data quality. A comparison with a smaller [CII] map taken with theHerschel/Heterodyne Instrument for the Far-Infrared spectrometer is presented.Conclusions.Large-scale [CII] mapping provides new insight into the kinematics of the ISM. The interaction between massive stars and the ISM is probed through [CII] observations. Spectrally resolving the [CII] emission is necessary to probe the microphysics induced by the feedback of massive stars. We show that certain heterodyne instrument data quality issues can be resolved using a spline-based technique, and better data correction routines allow for more efficient observing strategies.

Far-Infrared Line Diagnostics: Improving N/O Abundance Estimates for Dusty Galaxies

Abstract The nitrogen-to-oxygen (N/O) abundance ratio is an important diagnostic of galaxy evolution because the ratio is closely tied to the growth of metallicity and the star formation history in galaxies. Estimates for the N/O are traditionally made with optical lines that could suffer from extinction and excitation effects, so the N/O is arguably measured better through far-infrared (far-IR) fine-structure lines. Here we show that the [N iii]57 μm/[O iii]52 μm line ratio, denoted N3O3, is a physically robust probe of N/O. This parameter is insensitive to gas temperature and only weakly dependent on electron density. Although it has a dependence on the hardness of the ionizing radiation field, we show that it is well corrected when the [Ne iii]15.5 μm/[Ne ii]12.8 μm line ratio is included. We verify the method, and characterize its intrinsic uncertainties by comparing the results to photoionization models. We then apply our method to a sample of nearby galaxies using new observations obtained with SOFIA/FIFI-LS in combination with available Herschel/PACS data, and the results are compared with optical N/O estimates. We find evidence for a systematic offset between the far-IR and optically derived N/O. We argue that the likely reason is that our far-IR method is biased toward younger and denser H ii regions, while the optical methods are biased toward older H ii regions as well as diffuse ionized gas. This work provides a local template for studies of the abundance of interstellar medium in the early Universe.

A Herschel Mapping of , and Lines from the Circumnuclear Region of M31

Abstract The circumnuclear region of M31, consisting of multiphase interstellar media, provides a close-up view of the interaction of the central supermassive black hole and surrounding materials. Far-infrared (FIR) fine-structure lines and their flux ratios can be used as diagnostics of physical properties of the neutral gas in this region. Here we present the first FIR spectroscopic mapping of the circumnuclear region of M31 in [C ii] 158 μm, [O i] 63 μm, and [O iii] 88 μm lines with the Herschel Space Observatory, covering an ∼500 × 500 pc (2′ × 2′) field. Significant emissions of all three lines are detected along the so-called nuclear spiral across the central kiloparsec of M31. The velocity field under a spatial resolution of ∼50 pc of the three lines are broadly consistent and also consistent with previous CO(3–2) line observations in the central region. Combined with existing [C ii] and CO(3–2) observations of five other fields targeting the disk, we derived the radial distribution of [C ii]/CO(3–2) flux ratio, and found that this ratio is higher in the center than in the disk, indicating a low gas density and strong radiation field in the central region. We also found that the [C ii]/FIR ratio in the central region is 5.4 (±0.8) × 10−3, which exhibits an increasing trend with the galactocentric radius, suggesting an increasing contribution from the old stellar population to dust heating toward the center.

FEEDBACK: a SOFIA Legacy Program to Study Stellar Feedback in Regions of Massive Star Formation

FEEDBACK is a SOFIA (Stratospheric Observatory for Infrared Astronomy) legacy program dedicated to study the interaction of massive stars with their environment. It performs a survey of 11 galactic high mass star-forming regions in the 158 μm (1.9 THz) line of [C ii] and the 63 μm (4.7 THz) line of [O i]. We employ the 14 pixel Low Frequency Array and 7 pixel High Frequency Array upGREAT heterodyne instrument to spectrally resolve (0.24 MHz) these far-infrared fine structure lines. With a total observing time of 96h, we will cover ∼6700 arcmin2 at 14.″1) angular resolution for the [C ii] line and 6.″3 for the [O i] line. The observations started in spring 2019 (Cycle 7). Our aim is to understand the dynamics in regions dominated by different feedback processes from massive stars such as stellar winds, thermal expansion, and radiation pressure, and to quantify the mechanical energy injection and radiative heating efficiency. This is an important science topic because feedback of massive stars on their environment regulates the physical conditions and sets the emission characteristics in the interstellar medium (ISM), influences the star formation activity through molecular cloud dissolution and compression processes, and drives the evolution of the ISM in galaxies. The [C ii] line provides the kinematics of the gas and is one of the dominant cooling lines of gas for low to moderate densities and UV fields. The [O i] line traces warm and high-density gas, excited in photodissociations regions with a strong UV field or by shocks. The source sample spans a broad range in stellar characteristics from single OB stars, to small groups of O stars, to rich young stellar clusters, to ministarburst complexes. It contains well-known targets such as Aquila, the Cygnus X region, M16, M17, NGC7538, NGC6334, Vela, and W43 as well as a selection of H ii region bubbles, namely RCW49, RCW79, and RCW120. These [C ii] maps, together with the less explored [O i] 63 μm line, provide an outstanding database for the community. They will be made publically available and will trigger further studies and follow-up observations.

The CO-dark molecular gas mass in 30 Doradus

ABSTRACT Determining the efficiency with which gas is converted into stars in galaxies requires an accurate determination of the total reservoir of molecular gas mass. However, despite being the most abundant molecule in the Universe, H2 is challenging to detect through direct observations and indirect methods have to be used to estimate the total molecular gas reservoir. These are often based on scaling relations from tracers such as CO or dust, and are generally calibrated in the Milky Way. Yet, evidence that these scaling relations are environmentally dependent is growing. In particular, the commonly used CO-to-H2 conversion factor (XCO) is expected to be higher in metal-poor and/or strongly UV-irradiated environments. We use new SOFIA/FIFI-LS observations of far-infrared fine-structure lines from the ionized and neutral gas and the Meudon photodissociation region model to constrain the physical properties and the structure of the gas in the massive star-forming region of 30 Doradus in the Large Magellanic Cloud, and determine the spatially resolved distribution of the total reservoir of molecular gas in the proximity of the young massive cluster R136. We compare this value with the molecular gas mass inferred from ground-based CO observations and dust-based estimates to quantify the impact of this extreme environment on commonly used tracers of the molecular gas. We find that the strong radiation field combined with the half-solar metallicity of the surrounding gas is responsible for a large reservoir of ‘CO-dark’ molecular gas, leaving a large fraction of the total H2 gas (≳75 per cent) undetected when adopting a standard XCO factor in this massive star-forming region.

C II] 158 μm self-absorption and optical depth effects

Context. The [C II] 158 μm far-infrared fine-structure line is one of the most important cooling lines of the star-forming interstellar medium (ISM). It is used as a tracer of star formation efficiency in external galaxies and to study feedback effects in parental clouds. High spectral resolution observations have shown complex structures in the line profiles of the [C II] emission. Aims. Our aim is to determine whether the complex profiles observed in [12C II] are due to individual velocity components along the line-of-sight or to self-absorption based on a comparison of the [12C II] and isotopic [13C II] line profiles. Methods. Deep integrations with the SOFIA/upGREAT 7-pixel array receiver in the sources of M43, Horsehead PDR, Monoceros R2, and M17 SW allow for the detection of optically thin [13C II] emission lines, along with the [12C II] emission lines, with a high signal-to-noise ratio. We first derived the [12C II] optical depth and the [C II] column density from a single component model. However, the complex line profiles observed require a double layer model with an emitting background and an absorbing foreground. A multi-component velocity fit allows us to derive the physical conditions of the [C II] gas: column density and excitation temperature. Results. We find moderate to high [12C II] optical depths in all four sources and self-absorption of [12C II] in Mon R2 and M17 SW. The high column density of the warm background emission corresponds to an equivalent Av of up to 41 mag. The foreground absorption requires substantial column densities of cold and dense [C II] gas, with an equivalent Av ranging up to about 13 mag. Conclusions. The column density of the warm background material requires multiple photon-dominated region surfaces stacked along the line of sight and in velocity. The substantial column density of dense and cold foreground [C II] gas detected in absorption cannot be explained with any known scenario and we can only speculate on its origins.

A dense, solar metallicity ISM in the z = 4.2 dusty star-forming galaxy SPT 0418−47

We present a study of six far-infrared fine structure lines in the z = 4.225 lensed dusty star-forming galaxy SPT 0418−47 to probe the physical conditions of its interstellar medium (ISM). In particular, we report Atacama Pathfinder EXperiment (APEX) detections of the [OI] 145 μm and [OIII] 88 μm lines and Atacama Compact Array (ACA) detections of the [NII] 122 and 205 μm lines. The [OI] 145 μm/[CII] 158 μm line ratio is ∼5× higher compared to the average of local galaxies. We interpret this as evidence that the ISM is dominated by photo-dissociation regions with high gas densities. The line ratios, and in particular those of [OIII] 88 μm and [NII] 122 μm imply that the ISM in SPT 0418−47 is already chemically enriched to nearly solar metallicity. While the strong gravitational amplification was required to detect these lines with APEX, larger samples can be observed with the Atacama Large Millimeter/submillimeter Array (ALMA), and should allow observers to determine if the dense, solar metallicity ISM is common among these highly star-forming galaxies.

CO and Fine-structure Lines Reveal Low Metallicity in a Stellar-mass-rich Galaxy at z ∼ 1?

Abstract We present detections of the CO(4–3) and [C i] 609 μm spectral lines, as well as the dust continuum at 480.5 GHz (rest frame), in 3C 368, a Fanaroff–Riley class II (FR-II) galaxy at redshift (z) 1.131. 3C 368 has a large stellar mass, ∼ 3.6 × 1011 M ⊙, and is undergoing an episode of vigorous star formation, at a rate of ∼ 350 M ⊙ yr−1, and active galactic nucleus activity, with radio-emitting lobes extended over ∼ 73 kpc. Our observations allow us to inventory the molecular-gas reservoirs in 3C 368 by applying three independent methods: (1) using the CO(4–3)-line luminosity, excitation state of the gas, and an α CO conversion factor, (2) scaling from the [C i]-line luminosity, and (3) adopting a gas-to-dust conversion factor. We also present gas-phase metallicity estimates in this source, both using far-infrared fine-structure lines together with radio free–free continuum emission and independently employing the optical [O iii] 5007 Å and [O ii] 3727 Å lines (R 23 method). Both methods agree on a subsolar gas-phase metallicity of ∼ 0.3 Z ⊙. Intriguingly, comparing the molecular-gas mass estimated using this subsolar metallicity, M gas ∼ 6.4 × 1010 M ⊙, to dust-mass estimates from multicomponent spectral energy distribution modeling, M dust ∼ 1.4 × 108 M ⊙, yields a gas-to-dust ratio within ∼ 15% of the accepted value for a metallicity of 0.3 Z ⊙. The derived gas mass puts 3C 368 on a par with other galaxies at z ∼ 1 in terms of specific star formation rate and gas fraction. However, it does not explain how a galaxy can amass such a large stellar population while maintaining such a low gas-phase metallicity. Perhaps 3C 368 has recently undergone a merger, accreting pristine molecular gas from an external source.

The ‘Red Radio Ring’: ionized and molecular gas in a starburst/active galactic nucleus at z ∼ 2.55

ABSTRACT We report the detection of the far-infrared (FIR) fine-structure line of singly ionized nitrogen, [N ii] 205 $\mu$m , within the peak epoch of galaxy assembly, from a strongly lensed galaxy, hereafter ‘The Red Radio Ring’; the RRR, at z = 2.55. We combine new observations of the ground-state and mid-J transitions of CO (Jup = 1, 5, 8), and the FIR spectral energy distribution (SED), to explore the multiphase interstellar medium (ISM) properties of the RRR. All line profiles suggest that the H ii regions, traced by [N ii] 205 $\mu$m , and the (diffuse and dense) molecular gas, traced by CO, are cospatial when averaged over kpc-sized regions. Using its mid-IR-to-millimetre (mm) SED, we derive a non-negligible dust attenuation of the [N ii] 205 $\mu$m line emission. Assuming a uniform dust screen approximation results a mean molecular gas column density &amp;gt;1024 cm−2, with a molecular gas-to-dust mass ratio of 100. It is clear that dust attenuation corrections should be accounted for when studying FIR fine-structure lines in such systems. The attenuation corrected ratio of $L_{\rm N\,{\small II}205} / L_{\rm IR(8\!-\!1000\, \mu m)} = 2.7 \times 10^{-4}$ is consistent with the dispersion of local and z &amp;gt; 4 SFGs. We find that the lower limit, [N ii] 205 $\mu$m -based star formation rate (SFR) is less than the IR-derived SFR by a factor of 4. Finally, the dust SED, CO line SED, and $L_{\rm N\,{\small II}205}$ line-to-IR luminosity ratio of the RRR is consistent with a starburst-powered ISM.

Properties of galaxies at z ≈ 7 – 9 revealed by ALMA

AbstractUnderstanding properties of galaxies in the epoch of reionization (EoR) is a frontier in the modern astronomy. With the advent of ALMA, it has become possible to detect far-infrared fine structure lines (e.g. [CII] 158 μm and [OIII] 88 μm) and dust continuum emission in star-forming galaxies in the EoR. Among these lines, our team is focusing on [OIII] 88 μm observations in high-z galaxies. After the first detection of [OIII] in the epoch of reionization (EoR) in 2016 from our team at z = 7.21, there are now more than ten [OIII] detections at z &gt; 6 up to z = 9.11. Interestingly, high-z galaxies typically have very high [OIII]-to-[CII] luminosity ratio ranging from 3 to 12 or higher, demonstrating [OIII] is a powerful tracer at high-z. The high luminosity ratios may imply that high-z galaxies have low gas-phase metallicity and/or high ionization states.

Resolving Star Formation on Subkiloparsec Scales in the High-redshift Galaxy SDP.11 Using Gravitational Lensing*

Abstract We investigate the properties of the interstellar medium, star formation, and the current-day stellar population in the strongly lensed star-forming galaxy H-ATLAS J091043.1-000321 (SDP.11), at z = 1.7830, using new Herschel and Atacama Large Millimeter/submillimeter Array (ALMA) observations of far-infrared fine-structure lines of carbon, oxygen, and nitrogen. We report detections of the [O iii] 52 μm, [N iii] 57 μm, and [O i] 63 μm lines from Herschel/PACS, and present high-resolution imaging of the [C ii] 158 μm line, and underlying continuum, using ALMA. We resolve the [C ii] line emission into two spatially offset Einstein rings, tracing the red and blue velocity components of the line, in the ALMA/Band 9 observations at 0.″2 resolution. The values seen in the [C ii]/far-infrared (FIR) ratio map, as low as ∼0.02% at the peak of the dust continuum, are similar to those of local ULIRGs, suggesting an intense starburst in this source. This is consistent with the high intrinsic FIR luminosity (∼3 × 1012 L ⊙), ∼16 Myr gas depletion timescale, and ≲8 Myr timescale since the last starburst episode, estimated from the hardness of the UV radiation field. By applying gravitational lensing models to the visibilities in the uv-plane, we find that the lensing magnification factor varies by a factor of two across SDP.11, affecting the observed line profiles. After correcting for the effects of differential lensing, a symmetric line profile is recovered, suggesting that the starburst present here may not be the result of a major merger, as is the case for local ULIRGs, but instead could be powered by star formation activity spread across a 3–5 kpc rotating disk.

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.

SHINING, A Survey of Far-infrared Lines in Nearby Galaxies. I. Survey Description, Observational Trends, and Line Diagnostics

Abstract We use the Herschel/PACS spectrometer to study the global and spatially resolved far-infrared (FIR) fine-structure line emission in a sample of 52 galaxies that constitute the SHINING survey. These galaxies include star-forming, active-galactic nuclei (AGNs), and luminous infrared galaxies (LIRGs). We find an increasing number of galaxies (and kiloparsec-size regions within galaxies) with low line-to-FIR continuum ratios as a function of increasing FIR luminosity (L FIR), dust infrared color, L FIR to molecular gas mass ratio (L FIR/M mol), and FIR surface brightness (ΣFIR). The correlations between the [C ii]/FIR or [O i]/FIR ratios with ΣFIR are remarkably tight (∼0.3 dex scatter over almost four orders of magnitude in ΣFIR). We observe that galaxies with and ΣFIR ≳ 1011 L ⊙ kpc−2 tend to have weak fine-structure line-to-FIR continuum ratios, and that LIRGs with infrared sizes ≳1 kpc have line-to-FIR ratios comparable to those observed in typical star-forming galaxies. We analyze the physical mechanisms driving these trends in Paper II. The combined analysis of the [C ii], [N ii] 122 μm, and [O iii] 88 μm lines reveals that the fraction of the [C ii] line emission that arises from neutral gas increases from 60% to 90% in the most active star-forming regions and that the emission originating in the ionized gas is associated with low-ionization, diffuse gas rather than with dense gas in H ii regions. Finally, we report the global and spatially resolved line fluxes of the SHINING galaxies to enable the comparison and planning of future local and high-z studies.