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This is the fourth paper exploring the infrared properties of giant H ii regions with the FORCAST instrument on the Stratospheric Observatory For Infrared Astronomy (SOFIA). Our survey utilizes the census of 56 Milky Way giant H ii regions identified by Conti & Crowther, and in this paper we present the 20 and 37 ÎĽ m imaging data we obtained from SOFIA for sources Sgr D and W42. Based upon the SOFIA data and other multiwavelength data, we derive and discuss the detailed physical properties of the individual compact sources and subregions as well as the large-scale properties of Sgr D and W42. However, improved measurements have revealed much closer distances to both regions than previously believed, and consequently, both sources are not powerful enough to be considered giant H ii regions any longer. Motivated by this, we revisit the census of giant H ii regions, performing a search of the last two decades of literature to update each source with the most recent and/or most accurate distance measurements. Based on these new distance estimates, we determine that 14 sources in total (25%) are at sufficiently reliable and closer distances that they are not powerful enough to be considered giant H ii regions. We briefly discuss the observational and physical characteristics specific to Sgr D and W42 and show that they have properties distinct from the giant H ii regions previously studied as a part of this survey. [ABSTRACT FROM AUTHOR]

We describe the data processing of the Survey on extragALactic magnetiSm with SOFIA (SALSA Legacy Program). This first data release presents 33% (51.34 hr out of 155.7 hr, including overheads) of the total awarded time from 2020 January to 2021 December. Our observations were performed using the newly implemented on-the-fly mapping (OTFMAP) technique in the polarimetric mode. We present the pipeline steps to obtain homogeneously reduced high-level data products of polarimetric maps of galaxies for use in scientific analysis. Our approach has a general design and can be applied to sources smaller than the field of view of the HAWC+ array in any given band. We estimate that the OTFMAP polarimetric mode offers a reduction of observing overheads by a factor 2.34 and an improvement in sensitivity by a factor 1.80 when compared to the same on-source time polarimetric observations using the chopping and nodding mode. The OTFMAP is a significant optimization of the polarimetric mode of HAWC+, as it ultimately reduces the cost of operations of HAWC+/SOFIA by increasing the science collected per hour of observation up to an overall factor of 2.49. The OTFMAP polarimetric mode is the standard observing strategy of SALSA. The results and quantitative analysis of this first data release are presented in Papers IV and V of the series. [ABSTRACT FROM AUTHOR]

We report SOFIA/GREAT observations of high-J CO lines and [C ii ] observations of the super star cluster candidate H72.97-69.39 in the Large Magellanic Cloud (LMC), which is in its very early formation stage. We use our observations to determine if shocks are heating the gas or if photon-dominated regions (PDRs) are being heated by local far-UV radiation. We use a PDR model and a shock model to determine whether the CO and [C ii ] lines arise from PDRs or shocks. We can reproduce the observed high-J CO and [C ii ] emission with a clumpy PDR model with the following properties: a density of 104.7 cm−3, a mass of 104M⊙, and UV radiation of 103.5 in units of Draine field. Comparison with the ALMA beam-filling factor suggests a higher density within the uncertainty of the fit. We find the lower-limit [C ii ]/total infrared (TIR) ratio (ϵ) traced by [C ii ]/TIR to be 0.026%, lower than other known young star-forming regions in the LMC. Our shock models may explain the CO (16−15) and CO (11−10) emission lines with shock velocity of 8–11 km s−1, pre-shock density of 104–105 cm−3, and GUV = 0 in units of Draine field. However, the [C ii ] line emission cannot be explained by a shock model, thus it is originating in a different gas component. Observations of [O i ] 63 μm predicted to be 1.1 × 10−13 W m−2 by PDR models and 7.8 × 10−15 W m−2 by shock models will help distinguish between the PDR and shock scenarios. [ABSTRACT FROM AUTHOR]

M82 is an archetypal starburst galaxy in the local Universe. The central burst of star formation, thought to be triggered by M82's interaction with other members in the M81 group, is driving a multiphase galaxy-scale wind away from the plane of the disk that has been studied across the electromagnetic spectrum. Here, we present new velocity-resolved observations of the [C ii ] 158 μ m line in the central disk and the southern outflow of M82 using the upGREAT instrument on board SOFIA. We also report the first detections of velocity-resolved (Δ V = 10 km s−1) [C ii ] emission in the outflow of M82 at projected distances of ≈1–2 kpc south of the galaxy center. We compare the [C ii ] line profiles to observations of CO and H i and find that likely the majority (>55%) of the [C ii ] emission in the outflow is associated with the neutral atomic medium. We find that the fraction of [C ii ] actually outflowing from M82 is small compared to the bulk gas outside the midplane (which may be in a halo or tidal streamers), which has important implications for observations of [C ii ] outflows at higher redshift. Finally, by comparing the observed ratio of the [C ii ] and CO intensities to models of photodissociation regions, we estimate that the far-ultraviolet (FUV) radiation field in the disk is ∼103.5 G 0, in agreement with previous estimates. In the outflow, however, the FUV radiation field is 2–3 orders of magnitudes lower, which may explain the high fraction of [C ii ] arising from the neutral medium in the wind. [ABSTRACT FROM AUTHOR]

WR 137 (HD 192641) is a binary system consisting of a carbon-rich Wolf–Rayet (W-R) star and an Oe companion star in a 13 yr orbit. Near periastron, the winds of the two stars collide and form carbonaceous dust. We obtained three mid-infrared grism spectra of the system with SOFIA and FORCAST during the last year of SOFIA's operations in 2021 July, 2021 February, and 2022 May (Cycle 9). Within these spectra, we have identified several wind lines from He i, He ii, C iii, and C iv that are emitted from the W-R wind as well as a weak emission feature around 6.3–6.4 μ m that may have shifted its peak flux from 6.29 to 6.41 μ m through this time period. The weak feature grew as the continuum dust emission grew while the W-R emission appeared to decline due to lower contrast with the continuum. Furthermore, we observe that the peak of the feature shifts to redder wavelengths during the observations. We compare this feature to the unidentified infrared feature and other emission lines identified in dusty carbon-rich W-R (WC) binaries. For WR 137, we speculate that mixing of the winds in the system with the Oe star's disk is important for starting the dust formation and that it is less important as dust formation continues. Previous infrared photometry shows "minieruptions" of dust production, which could then be explained with variations of the Oe star disk. [ABSTRACT FROM AUTHOR]

We present the analysis of the magnetic field (B -field) structure of galaxies measured with far-infrared (FIR) and radio (3 and 6 cm) polarimetric observations. We use the first data release of the Survey of extragALactic magnetiSm with SOFIA of 14 nearby ( < 20 Mpc) galaxies with resolved ( 5 ″ – 18 ″ ; 90 pc–1 kpc) imaging polarimetric observations using SOFIA/HAWC+ from 53 to 214 μ m. We compute the magnetic pitch-angle ( Ψ B ) profiles as a function of the galactocentric radius. We introduce a new magnetic alignment parameter (ζ) to estimate the disordered-to-ordered ratio of spiral B -fields. We find FIR and radio wavelengths to not generally trace the same B -field morphology in galaxies. The Ψ B profiles tend to be more ordered across all galactocentric radii in radio ( ζ 6 cm = 0.93 ± 0.03) than in FIR ( ζ 154 μ m = 0.84 ± 0.14). For spiral galaxies, FIR B -fields are 2%–75% more turbulent than the radio B -fields. For starburst galaxies, we find that FIR polarization is a better tracer of the B -fields along the galactic outflows than radio polarization. Our results suggest that the B -fields associated with dense, dusty, turbulent star-forming regions (those traced at FIR) are less ordered than warmer, less dense regions (those traced at radio) of the interstellar medium. The FIR B -fields seem to be more sensitive to the activity of the star-forming regions and molecular clouds within a vertical height of a few hundred parsecs in the disk of spiral galaxies than the radio B -fields. [ABSTRACT FROM AUTHOR]

The heart of the Large Magellanic Cloud, 30 Doradus, is a complex region with a clear core-halo structure. Feedback from the stellar cluster R136 has been shown to be the main source of energy creating multiple parsec-scale expanding-shells in the outer region, and carving a nebula core in the proximity of the ionization source. We present the morphology and strength of the magnetic fields (B -fields) of 30 Doradus inferred from the far-infrared polarimetric observations by SOFIA/HAWC+ at 89, 154, and 214 μ m. The B -field morphology is complex, showing bending structures around R136. In addition, we use high spectral and angular resolution [C ii ] observations from SOFIA/GREAT and CO(2-1) from APEX. The kinematic structure of the region correlates with the B -field morphology and shows evidence of multiple expanding-shells. Our B -field strength maps, estimated using the Davis–Chandrasekhar–Fermi method and structure-function, show variations across the cloud within a maximum of 600, 450, and 350 μ G at 89, 154, and 214 μ m, respectively. We estimated that the majority of the 30 Doradus clouds are subcritical and sub-Alfvénic. The probability distribution function of the gas density shows that the turbulence is mainly compressively driven, while the plasma beta parameter indicates supersonic turbulence. We show that the B -field is sufficient to hold the cloud structure integrity under feedback from R136. We suggest that supersonic compressive turbulence enables the local gravitational collapse and triggers a new generation of stars to form. The velocity gradient technique using [C ii ] and CO(2-1) is likely to confirm these suggestions. [ABSTRACT FROM AUTHOR]

We present the first data release of the Survey on extragALactic magnetiSm with SOFIA (SALSA Legacy Program) with a set of 14 nearby (<20 Mpc) galaxies with resolved imaging polarimetric observations using HAWC+ from 53 to 214 ÎĽ m at a resolution of 5″â€"18″ (90 pcâ€"1 kpc). We introduce the definitions of and background on extragalactic magnetism and present the scientific motivation and sample selection of the program. Here we focus on the general trends in the emissive polarization fraction. Far-infrared polarimetric observations trace the thermal polarized emission of magnetically aligned dust grains across the galaxy disks with polarization fractions of P = 0%â€"15% in the cold, T d = [19, 48] K, and dense, log 10 (N H I + H 2 [ cm âˆ' 2 ]) = [ 19.96 , 22.91 ] , interstellar medium. The spiral galaxies show a median 〈 P 154 ÎĽ m〉 = 3.3% ± 0.9% across the disks. We report the first polarized spectrum of starburst galaxies showing a minimum within 89â€"154 ÎĽ m. The falling 53â€"154 ÎĽ m polarized spectrum may be due to a decrease in the dust grain alignment efficiency produced by variations in dust temperatures along the line of sight in the galactic outflow. We find that the starburst galaxies and the star-forming regions within normal galaxies have the lowest polarization fractions. We find that 50% (seven out of 14) of the galaxies require a broken power law in the P âˆ' N H I + H 2 and P âˆ' T d relations with three different trends. Group 1 has a relative increase of anisotropic random B-fields produced by compression or shear of B-fields in the galactic outflows, starburst rings, and inner bars of galaxies, and groups 2 and 3 have a relative increase of isotropic random B-fields driven by star-forming regions in the spiral arms and/or an increase of dust grain alignment efficiency caused by shock-driven regions or evolutionary stages of a galaxy. [ABSTRACT FROM AUTHOR]

We present a study of the relative orientation between the magnetic field and elongated cloud structures for the ρ Oph A and ρ Oph E regions in L1688 in the Ophiuchus molecular cloud. Combining inferred magnetic field orientation from HAWC+ 154 μm observations of polarized thermal emission with column density maps created using Herschel submillimeter observations, we find consistent perpendicular relative alignment at scales of 0.02 pc (33.″6 at d ≈ 137 pc) using the histogram of relative orientations (HRO) technique. This supports the conclusions of previous work using Planck polarimetry and extends the results to higher column densities. Combining this HAWC+ HRO analysis with a new Planck HRO analysis of L1688, the transition from parallel to perpendicular alignment in L1688 is observed to occur at a molecular hydrogen column density of approximately 1021.7 cm−2. This value for the alignment transition column density agrees well with values found for nearby clouds via previous studies using only Planck observations. Using existing turbulent, magnetohydrodynamic simulations of molecular clouds formed by colliding flows as a model for L1688, we conclude that the molecular hydrogen volume density associated with this transition is approximately ∼104 cm−3. We discuss the limitations of our analysis, including incomplete sampling of the dense regions in L1688 by HAWC+. [ABSTRACT FROM AUTHOR]

The alignment of dust grains with the ambient magnetic field produces polarization of starlight as well as thermal dust emission. Using the archival SOFIA/HAWC+ polarimetric data observed toward the ρ Ophiuchus (Oph) A cloud hosted by a B star at 89 and 154 μm, we find that the fractional polarization of thermal dust emission first increases with the grain temperature and then decreases once the grain temperature exceeds ≃25–32 K. The latter trend differs from the prediction of the popular RAdiative Torques (RATs) alignment theory, which implies a monotonic increase of the polarization fraction with the grain temperature. We perform numerical modeling of polarized dust emission for the ρ Oph-A cloud and calculate the degree of dust polarization by simultaneously considering the dust grain alignment and rotational disruption by RATs. Our modeling results could successfully reproduce both the rising and declining trends of the observational data. Moreover, we show that the alignment of only silicate grains or a mixture of silicate–carbon grains within a composite structure can reproduce the observational trends, assuming that all dust grains follow a power-law size distribution. Although there are a number of simplifications and limitations to our modeling, our results suggest grains in the ρ Oph-A cloud have a composite structure, and the grain size distribution has a steeper slope than the standard size distribution for the interstellar medium. Combination of SOFIA/HAWC+ data with JCMT observations 450 and 850 μm would be useful to test the proposed scenario based on grain alignment and disruption by RATs. [ABSTRACT FROM AUTHOR]

We present SOFIA/FIFI-LS observations of the [C ii ] 158 μm cooling line across the nearby spiral galaxy NGC 6946. We combine these with UV, IR, CO, and H i data to compare [C ii ] emission to dust properties, star formation rate (SFR), H2, and H i at 560 pc scales via stacking by environment (spiral arms, interarm, and center), radial profiles, and individual, beam-sized measurements. We attribute 73% of the [C ii ] luminosity to arms, and 19% and 8% to the center and interarm region, respectively. [C ii ]/TIR, [C ii ]/CO, and [C ii ]/PAH radial profiles are largely constant, but rise at large radii (≳ 8 kpc) and drop in the center ("[C ii ] deficit"). This increase at large radii and the observed decline with the 70 μm/100 μm dust color are likely driven by radiation field hardness. We find a near proportional [C ii ]–SFR scaling relation for beam-sized regions, though the exact scaling depends on methodology. [C ii ] also becomes increasingly luminous relative to CO at low SFR (interarm or large radii), likely indicating more efficient photodissociation of CO and emphasizing the importance of [C ii ] as an H2 and SFR tracer in such regimes. Finally, based on the observed [C ii ] and CO radial profiles and different models, we find αCO to increase with radius, in line with the observed metallicity gradient. The low αCO (galaxy average ≲ 2 M⊙ pc−2 (K km s−1)−1) and low [C ii ]/CO ratios (∼400 on average) imply little CO-dark gas across NGC 6946, in contrast to estimates in the Milky Way. [ABSTRACT FROM AUTHOR]

Here we present the results of an airborne 3–5.4 μm spectroscopic study of three young, carbon-rich planetary nebulae (PNs) IC 5117, PNG 093.9-00.1, and BD +30 3639. These observations were made using the grism spectroscopy mode of the First Light Infrared TEst CAMera (FLITECAM) instrument during airborne science operations on board NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA). The goal of this study is to characterize the 3.3 and 5.25 μm polycyclic aromatic hydrocarbon (PAH) dust emission in PNs and study the evolution of PAH features within evolved stars before their incorporation into new stellar systems in star-forming regions. Targets were selected from Infrared Astronomical Satellite, Kuiper Airborne Observatory and Infrared Space Observatory source lists, and were previously observed with FLITECAM on the 3 m Shane telescope at Lick Observatory to allow direct comparison between the ground and airborne observations. We measure PAH emission equivalent width and central wavelength, classify the shape of the PAH emission, and determine the PAH/aliphatic ratio for each target. The 3.3 μm PAH emission feature is observed in all three objects. PNG 093.9-00.1 exhibits NGC 7027-like aliphatic emission in the 3.4–3.6 μm region while IC 5117 and BD +30 3639 exhibit less aliphatic structure. All three PNs additionally exhibit PAH emission at 5.25 μm. [ABSTRACT FROM AUTHOR]

We present the first complete, velocity-resolved [C ii ] 158 μm image of the M51 grand-design spiral galaxy, observed with the upgraded German Receiver for Astronomy at Terahertz frequencies instrument on the Stratospheric Observatory for Infrared Astronomy. [C ii ] is an important tracer of various phases of the interstellar medium (ISM), including ionized gas, neutral atomic, and diffuse molecular regions. We combine the [C ii ] data with H i , CO, 24 μm dust continuum, FUV, and NIR K-band observations to study the evolution of the ISM across M51's spiral arms in both position–position and position–velocity space. Our data show strong velocity gradients in H i , 12CO, and [C ii ] at the locations of the stellar arms (traced by K-band data) with a clear offset in position–velocity space between upstream molecular gas (traced by 12CO) and downstream star formation (traced by [C ii ]). We compare the observed position–velocity maps across the spiral arms with synthetic observations from numerical simulations of galaxies with both dynamical and quasi-stationary steady spiral arms that predict both tangential and radial velocities at the location of the spiral arms. We find that our observations, based on the observed velocity gradients and associated offset between CO and [C ii ], are consistent with the presence of shocks in the spiral arms in the inner parts of M51 and in the arm connecting the companion galaxy, M51b, in the outer parts of M51. [ABSTRACT FROM AUTHOR]

We have observed the massive protostar AFGL 2136 IRS 1 in multiple wavelength windows in the near- to mid-infrared at high (∼3 km s−1) spectral resolution using VLT+CRIRES, SOFIA+EXES, and Gemini North+TEXES. There is an abundance of H2O absorption lines from the ν1 and ν3 vibrational bands at 2.7 μm, from the ν2 vibrational band at 6.1 μm, and from pure rotational transitions near 10–13 μm. Analysis of state-specific column densities derived from the resolved absorption features reveals that an isothermal absorbing slab model is incapable of explaining the relative depths of different absorption features. In particular, the strongest absorption features are much weaker than expected, indicating optical depth effects resulting from the absorbing gas being well mixed with the warm dust that serves as the "background" continuum source at all observed wavelengths. The velocity at which the strongest H2O absorption occurs coincides with the velocity centroid along the minor axis of the compact disk in Keplerian rotation recently observed in H2O emission with ALMA. We postulate that the warm regions of this dust disk dominate the continuum emission at near- to mid-infrared wavelengths, and that H2O and several other molecules observed in absorption are probing this disk. Absorption line profiles are not symmetric, possibly indicating that the warm dust in the disk that produces the infrared continuum has a nonuniform distribution similar to the substructure observed in 1.3 mm continuum emission. [ABSTRACT FROM AUTHOR]

The Galactic Center contains some of the most extreme conditions for star formation in our Galaxy, as well as many other phenomena that are unique to this region. Given our relative proximity to the Galactic Center, we are able to study details of physical processes to a level that is simply not yet possible for more distant galaxies, yielding an otherwise inaccessible view of the nuclear region of a galaxy. We recently carried out a targeted imaging survey of mid-infrared bright portions of the Galactic Center at 25 and 37 μm using the FORCAST instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA). This survey was one of the inaugural Legacy Programs from SOFIA cycle 7, observing a total area of 403 arcmin2 (2180 pc2), including the Sgr A, B, and C complexes. Here we present an overview of the survey strategy, observations, and data reduction as an accompaniment to the initial public release of the survey data. We discuss interesting regions and features within the data, including extended features near the circumnuclear disk, structures in the Arched Filaments and Sickle H ii regions, and signs of embedded star formation in Sgr B2 and Sgr C. We also feature a handful of less well studied mid-infrared sources located between Sgr A and Sgr C that could be sites of relatively isolated star formation activity. Last, we discuss plans for subsequent publications and future data releases from the survey. [ABSTRACT FROM AUTHOR]

The Sgr A-H H ii regions are a collection of sources that may represent one of the most recent sites of star formation in the Galactic center (GC). Examining these H ii regions provides important information on the prevalence and distribution of massive stars in the region and may provide clues on the origin of other massive field stars throughout the GC. In this work, we present infrared imaging observations of the Sgr A-H H ii regions taken with the Faint Object Infrared Camera for the SOFIA Telescope (FORCAST) at 19.7, 25.2, 31.5, and 37.1 μm. These data provide high angular resolution maps (∼4″) of the sources, which allow us to study the morphology of spatially extended H ii regions in detail. The wavelength coverage of FORCAST also supplies important constraints on the dust mass, temperature, and luminosity of the sources. We produce spectral energy distribution models using DustEM to measure the luminosity and dust mass, which range from ∼3 × 103 to 6 × 105L⊙ and from ∼6 × 10−4 to 3 × 10−1 M⊙, respectively. Analysis of dust temperatures in the four spatially extended H ii regions (H1, H2, H3, and H5) shows that three of these objects require multiple heating sources, and we identify potential stellar candidates. We also compare the size and morphology of the H ii regions and demonstrate that the properties of H2 are consistent with in situ star formation. Lastly, we identify nine additional sources that may be part of the H complex and provide initial characterizations of their infrared emission. [ABSTRACT FROM AUTHOR]