Context . Sagittarius C (Sgr C) is a massive but relatively quiescent complex at the western edge of the Galaxy’s Central Molecular Zone (CMZ). While the Sgr B2 region on the opposite side of the CMZ has been extensively studied, Sgr C has received comparatively little attention. Aims . We aim to characterize the kinematics and physical state of the atomic gas in the Sgr C region using spatially and velocity-resolved emission from the [CII] line. This line traces the multi-phase gas that consists of the warm ionized medium, the warm and cold diffuse atomic medium, and the warm dense molecular gas and provides a complement to molecular line, dust, infrared, and radio observations. Methods . We present a fully sampled 74 × 47 pc map of the [CII] 158 µm fine-structure line toward Sgr C, observed with the upGREAT receiver onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) airborne observatory. The data feature a spatial resolution of 0.55 pc and a spectral resolution of 1 km s −1 . We analyzed these observations in conjunction with ancillary maps of the J = 2–1 transition of CO and its isotopologues observed with the PI230 receiver at the Atacama Pathfinder Experiment (APEX) telescope. Results . [CII] emission is widespread throughout the region and shows a continuous structure that extends from Sgr A to Sgr C with a complex morphology. The majority of the emission arises from gas at negative radial velocities, consistent with the direction of Galactic rotation. The most prominent feature is the giant Sgr C HII region, where [CII] reveals an expanding, ring-like shell structure interpreted as a photodissociation region (PDR). We modeled the shell’s kinematics and derived an expansion velocity of ∼23 km s −1 and a dynamical age of ∼0.13 Myr. Our analysis suggests that stellar winds from the known massive stars are likely insufficient to power the observed expansion; this points toward alternative driving mechanisms such as a buried supernova. We find a striking spatial association between this shell and a nonthermal radio filament, which provides evidence that the shell’s expansion into the surrounding medium has triggered high-mass star formation at its edge.

After having adapted the Smithsonian Astrophysical Observatory (SAO) atmospheric phase monitoring system that was developed for the submillimeter array (SMA), IRAM is currently refining the concept for a future permanent implementation on the northern extended millimeter array (NOEMA) site. This system is based on commercial low noise block attached to a satellite dish that receive signal from geostationary satellites. The baselines (one so far) amplitude and phase are processed to provide real-time statistical data in the direction of the satellite and give information about the atmospheric phase distortion experienced by the NOEMA interferometer antennas.

4GREAT is an extension of the German receiver for astronomy at terahertz frequencies (GREAT) operated aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The spectrometer comprises four different detector bands and their associated subsystems for simultaneous and fully independent science operation. All detector beams are coaligned on the sky. The frequency bands of 4GREAT cover 491-635, 890-1090, 1240-1525, and 2490-2590 GHz, respectively. This article presents the design and characterization of the instrument, and its in-flight performance. The first light of 4GREAT was on June 2018. It has been offered to the interested SOFIA communities starting with observing cycle 6.

We report SOFIA-upGREAT spectroscopic imaging of the [C ii ] 158 μ m spectral line, as well as a number of [O i ] 63 μ m spectra, across a 67 × 45 pc field toward the Sgr B region in our Galactic center. The fully sampled and velocity-resolved [C ii ] images have 0.55 pc spatial and 1 km s −1 velocity resolutions. We find that Sgr B extends as a coherent structure spanning some 34 pc along the Galactic plane. Bright [C ii ] emission encompasses Sgr B1 (G0.5–0.0) and the G 0.6–0.0 H ii region and passes behind and beyond the luminous star-forming cores toward Sgr B2 (G0.7–0.0). Sgr B is a major contributor to the entire Galactic center’s [C ii ] luminosity, with surface brightness comparable to [C ii ] from the Arches region. [C ii ], 70 μ m, and 20 cm emissions share nearly identical spatial distributions. Combined with the lack of [C ii ] self-absorption, this indicates that these probes trace UV on the near surfaces of more extended clouds visible in CO isotopologues and 160 μ m continuum. Stars from regions of local star formation likely dominate the UV field. Photodissociation regions and H ii regions contribute similar amounts of [C ii ] flux. The extreme star formation cores of Sgr B2 contribute negligible amounts to the total [C ii ] intensity from the Sgr B region. Velocity fields and association with a narrow dust lane indicate that they may have been produced in a local cloud–cloud collision. The cores are likely local analogs of the intense star formation regions where ideas to explain the “C + deficit” in ultraluminous galaxies can be tested.

The Heterodyne Receiver for Origins (HERO) is the first detailed study of a heterodyne focal plane array receiver for space applications. HERO gives the Origins Space Telescope the capability to observe at very high spectral resolution (R = 10(7)) over an unprecedentedly large far-infrared (FIR) wavelengths range (111 to 617 mu m) with high sensitivity, with simultaneous dual polarization and dual-frequency band operation. The design is based on prior successful heterodyne receivers, such as Heterodyne Instrument for the Far-Infrared/Herschel, but surpasses it by one to two orders of magnitude by exploiting the latest technological developments. Innovative components are used to keep the required satellite resources low and thus allowing for the first time a convincing design of a large format heterodyne array receiver for space. HERO on Origins is a unique tool to explore the FIR universe and extends the enormous potential of submillimeter astronomical spectroscopy into new areas of astronomical research. (C) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License.

3C 279 is an archetypal blazar with a prominent radio jet that show broadband flux density variability across the entire electromagnetic spectrum. We use an ultra-high angular resolution technique – global Very Long Baseline Interferometry (VLBI) at 1.3 mm (230 GHz) – to resolve the innermost jet of 3C 279 in order to study its fine-scale morphology close to the jet base where highly variableγ-ray emission is thought to originate, according to various models. The source was observed during four days in April 2017 with the Event Horizon Telescope at 230 GHz, including the phased Atacama Large Millimeter/submillimeter Array (ALMA), at an angular resolution of ∼20 μas (at a redshift ofz = 0.536 this corresponds to ∼0.13 pc ∼ 1700 Schwarzschild radii with a black hole massMBH = 8 × 108 M⊙). Imaging and model-fitting techniques were applied to the data to parameterize the fine-scale source structure and its variation. We find a multicomponent inner jet morphology with the northernmost component elongated perpendicular to the direction of the jet, as imaged at longer wavelengths. The elongated nuclear structure is consistent on all four observing days and across different imaging methods and model-fitting techniques, and therefore appears robust. Owing to its compactness and brightness, we associate the northern nuclear structure as the VLBI “core”. This morphology can be interpreted as either a broad resolved jet base or a spatially bent jet. We also find significant day-to-day variations in the closure phases, which appear most pronounced on the triangles with the longest baselines. Our analysis shows that this variation is related to a systematic change of the source structure. Two inner jet components move non-radially at apparent speeds of ∼15 cand ∼20 c(∼1.3 and ∼1.7 μas day−1, respectively), which more strongly supports the scenario of traveling shocks or instabilities in a bent, possibly rotating jet. The observed apparent speeds are also coincident with the 3C 279 large-scale jet kinematics observed at longer (cm) wavelengths, suggesting no significant jet acceleration between the 1.3 mm core and the outer jet. The intrinsic brightness temperature of the jet components are ≲1010K, a magnitude or more lower than typical values seen at ≥7 mm wavelengths. The low brightness temperature and morphological complexity suggest that the core region of 3C 279 becomes optically thin at short (mm) wavelengths.

Heterodyne spectroscopic instruments are currently the only practical technical approach for obtaining velocity-resolved spectra in the far infrared. Moreover, to produce the large-scale maps of molecular clouds envisioned for future missions, large-format (100’s pixels) array receivers are required, which is the focus of this whitepaper.

During the dawn of chemistry(1,2), when the temperature of the young Universe had fallen below some 4,000 kelvin, the ions of the light elements produced in Big Bang nucleosynthesis recombined in reverse order of their ionization potential. With their higher ionization potentials, the helium ions He2+ and He+ were the first to combine with free electrons, forming the first neutral atoms; the recombination of hydrogen followed. In this metal-free and low-density environment, neutral helium atoms formed the Universe's first molecular bond in the helium hydride ion HeH+ through radiative association with protons. As recombination progressed, the destruction of HeH+ created a path to the formation of molecular hydrogen. Despite its unquestioned importance in the evolution of the early Universe, the HeH+ ion has so far eluded unequivocal detection in interstellar space. In the laboratory the ion was discovered3 as long ago as 1925, but only in the late 1970s was the possibility that HeH+ might exist in local astrophysical plasmas discussed4-7. In particular, the conditions in planetary nebulae were shown to be suitable for producing potentially detectable column densities of HeH+. Here we report observations, based on advances in terahertz spectroscopy8,9 and a high-altitude observatory10, of the rotational ground-state transition of HeH+ at a wavelength of 149.1 micrometres in the planetary nebula NGC 7027. This confirmation of the existence of HeH+ in nearby interstellar space constrains our understanding of the chemical networks that control the formation of this molecular ion, in particular the rates of radiative association and dissociative recombination.

Massive stars inject mechanical and radiative energy into the surrounding environment, which stirs it up, heats the gas, produces cloud and intercloud phases in the interstellar medium, and disrupts molecular clouds (the birth sites of new stars1,2). Stellar winds, supernova explosions and ionization by ultraviolet photons control the lifetimes of molecular clouds3–7. Theoretical studies predict that momentum injection by radiation should dominate that by stellar winds8, but this has been difficult to assess observationally. Velocity-resolved large-scale images in the fine-structure line of ionized carbon ([C ii]) provide an observational diagnostic for the radiative energy input and the dynamics of the interstellar medium around massive stars. Here we report observations of a one-square-degree region (about 7 parsecs in diameter) of Orion molecular core 1—the region nearest to Earth that exhibits massive-star formation—at a resolution of 16 arcseconds (0.03 parsecs) in the [C ii] line at 1.9 terahertz (158 micrometres). The results reveal that the stellar wind originating from the massive star θ1 Orionis C has swept up the surrounding material to create a ‘bubble’ roughly four parsecs in diameter with a 2,600-solar-mass shell, which is expanding at 13 kilometres per second. This finding demonstrates that the mechanical energy from the stellar wind is converted very efficiently into kinetic energy of the shell and causes more disruption of the Orion molecular core 1 than do photo-ionization and evaporation or future supernova explosions. Wind from the most massive star in the Trapezium cluster in Orion has carved out a large and expanding cavity around the cluster, bounded by a thin, 2,600-solar-mass shell.

The Event Horizon Telescope (EHT) is a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth. At a nominal operating wavelength of 1.3 mm, EHT angular resolution (lambda/D) is 25 micro-as, which is sufficient to resolve nearby supermassive black hole candidates on spatial and temporal scales that correspond to their event horizons. With this capability, the EHT scientific goals are to probe general relativistic effects in the strong-field regime and to study accretion and relativistic jet formation near the black hole boundary. In this Letter we describe the system design of the EHT, detail the technology and instrumentation that enable observations, and provide measures of its performance. Meeting the EHT science objectives has required several key developments that have facilitated the robust extension of the VLBI technique to EHT observing wavelengths and the production of instrumentation that can be deployed on a heterogeneous array of existing telescopes and facilities. To meet sensitivity requirements, high-bandwidth digital systems were developed that process data at rates of 64 gigabit/s, exceeding those of currently operating cm-wavelength VLBI arrays by more than an order of magnitude. Associated improvements include the development of phasing systems at array facilities, new receiver installation at several sites, and the deployment of hydrogen maser frequency standards to ensure coherent data capture across the array. These efforts led to the coordination and execution of the first Global EHT observations in 2017 April, and to event-horizon-scale imaging of the supermassive black hole candidate in M87.

C. Risacher1,6∗, R. Güsten, J. Stutzki, H.-W. Hübers, R. Aladro, A. Bell, C. Buchbender, D. Büchel, T. Csengeri, C. Duran, U. U. Graf, R. D. Higgins, C. E. Honingh, K. Jacobs, M. Justen, B. Klein , M. Mertens, Y. Okada, A. Parikka, P. Pütz, N. Reyes, H. Richter, O. Ricken, D. Riquelme, N. Rothbart, N. Schneider, R. Simon, M. Wienold, H. Wiesemeyer, M. Ziebart, P. Fusco, S. Rosner and B. Wohler Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121, Bonn, Germany, crisache@mpifr.de I. Physikalisches Institut der Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany Institute of Optical Sensor Systems, German Aerospace Center (DLR), Rutherfordstr. 2, 12489 Berlin, Germany Departamento de Ingenieŕıa Eléctrica, Universidad de Chile, Santiago, Chile University of Applied Sciences Bonn-Rhein-Sieg, Sankt Augustin, 53757 Germany IRAM, 300 rue de la Piscine, 38406 Saint Martin d’Heres, France NASA Ames Research Center, Moffett Field, CA 94035, USA SETI Institute, Mountain View, CA 94043, USA

The HEterodyne Receiver for the Origins Space Telescope (HERO) is a proposed design for a heterodyne focal plane array for a large space mission. The Origins Space Telescope (OST) is one of the four missions selected to be studied by NASA for the 2020 Astronomy and Astrophysics Decadal survey. HERO is designed to observe the trail of water from the interstellar medium (ISM) to disks around protostars. In Concept 1, HERO provides continuous frequency coverage from 468 to 2700 GHz in fi e bands and a sixth band to cover 4700 GHz. Most bands include 2 x 64 pixels providing at least an order of magnitude higher mapping speeds than available with today's instruments. Receiver sensitivities are expected to be close to the quantum limit. HERO Concept 2, highly constrained by cost and denoted Little-HERO, includes four bands with continuous coverage from 486 to 2700 GHz and with focal plane arrays having only 2 x 9 pixels per band. Both of these THz receiver concepts will be described and the designs will be motivated by the science drivers, the space craft constraints and the latest technological developments. The HERO design builds on the highly successful Herschel/Heterodyne Instrument for the Far-Infrared, on Stratospheric Observatory for Far-Infrared Astronomy/upGREAT and many other heterodyne receivers, but surpasses these in terms of frequency coverage, array size and sensitivity, thanks to the latest technical advances. HERO can be considered an example of a new generation of heterodyne focal plane arrays for future space missions.

Context: L1630 in the Orion B molecular cloud, which includes the iconic Horsehead Nebula, illuminated by the star system σ Ori, is an example of a photodissociation region (PDR). In PDRs, stellar radiation impinges on the surface of dense material, often a molecular cloud, thereby inducing a complex network of chemical reactions and physical processes. Aims: Observations toward L1630 allow us to study the interplay between stellar radiation and a molecular cloud under relatively benign conditions, that is, intermediate densities and an intermediate UV radiation field. Contrary to the well-studied Orion Molecular Cloud 1 (OMC1), which hosts much harsher conditions, L1630 has little star formation. Our goal is to relate the [C ii] fine-structure line emission to the physical conditions predominant in L1630 and compare it to studies of OMC1. Methods: The [C ii] 158μm line emission of L1630 around the Horsehead Nebula, an area of 12′ × 17′, was observed using the upgraded German Receiver for Astronomy at Terahertz Frequencies (upGREAT) onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). Results: Of the [C ii] emission from the mapped area 95%, 13L , originates from the molecular cloud; the adjacent H ii region contributes only 5%, that is, 1L . From comparison with other data (CO (1-0)-line emission, far-infrared (FIR) continuum studies, emission from polycyclic aromatic hydrocarbons (PAHs)), we infer a gas density of the molecular cloud of nH ∼ 3 · 10 cm−3, with surface layers, including the Horsehead Nebula, having a density of up to nH ∼ 4 · 10 cm−3. The temperature of the surface gas is T ∼ 100 K. The average [C ii] cooling efficiency within the molecular cloud is 1.3 · 10−2. The fraction of the mass of the molecular cloud within the studied area that is traced by [C ii] is only 8%. Our PDR models are able to reproduce the FIR-[C ii] correlations and also the CO (1-0)-[C ii] correlations. Finally, we compare our results on the heating efficiency of the gas with theoretical studies of photoelectric heating by PAHs, clusters of PAHs, and very small grains, and find the heating efficiency to be lower than theoretically predicted, a continuation of the trend set by other observations. Conclusions: In L1630 only a small fraction of the gas mass is traced by [C ii]. Most of the [C ii] emission in the mapped area stems from PDR surfaces. The layered edge-on structure of the molecular cloud and limitations in spatial resolution put constraints on our ability to relate different tracers to each other and to the physical conditions. From our study, we conclude that the relation between [C ii] emission and physical conditions is likely to be more complicated than often assumed. The theoretical heating efficiency is higher than the one we calculate from the observed [C ii] emission in the L1630 molecular cloud.

We have combined emission from the 158 mu m fine structure transition of C+ observed with the GREAT and upGREAT instruments on SOFIA with 21 cm absorption spectra and visual extinction to characterize the diffuse interstellar clouds found along the lines of sight. The weak [C II] emission is consistent in velocity and line width with the strongest H I component produced by the cold neutral medium. The H I column density and kinetic temperature are known from the 21 cm data and, assuming a fractional abundance of ionized carbon, we calculate the volume density and thermal pressure of each source, which vary considerably, with 27 cm(-3) <= n(H-0) <= 210 cm(-3) considering only the atomic hydrogen along the lines of sight to be responsible for the C+, while 13 cm(-3) <= n(H-0 + H-2) <= 190 cm(-3) including the hydrogen in both forms. The thermal pressure varies widely with 1970 cm(-3) K <= P-th/k <= 10,440 cm(-3) K for H-0 alone and 750 cm(-3) K <= P-th/k <= 9360 cm(-3) K including both H-0 and H-2. The molecular hydrogen fraction varies between 0.10 and 0.67. Photoelectric heating is the dominant heating source, supplemented by a moderately enhanced cosmic ray ionization rate, constrained by the relatively low 45 K to 73 K gas temperatures of the clouds. The resulting thermal balance for the two lower-density clouds is satisfactory, but for the two higher-density clouds, the combined heating rate is insufficient to balance the observed C+ cooling.