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1. The GBT Diffuse Ionized Gas Survey (GDIGS): Discrete Sources

   https://doi.org/10.3847/1538-4357/acfc39  

   Linville, Dylan J.; Luisi, Matteo; Anderson, L. D.; Liu, Bin; Bania, T. M.; Balser, Dana S.; Wenger, Trey V.; Haffner, L. M.; Mascoop, J. L. (December 2023, The Astrophysical Journal)

   Abstract The Green Bank Telescope Diffuse Ionized Gas Survey (GDIGS) traces ionized gas in the Galactic midplane by observing radio recombination line (RRL) emission from 4 to 8 GHz. The nominal survey zone is 32.°3 >ℓ> −5°, ∣b∣ < 0.°5. Here, we analyze GDIGS Hnαionized gas emission toward discrete sources. Using GDIGS data, we identify the velocity of 35 Hiiregions that have multiple detected RRL velocity components. We identify and characterize RRL emission from 88 Hiiregions that previously lacked measured ionized gas velocities. We also identify and characterize RRL emission from eight locations that appear to be previously unidentified Hiiregions and 30 locations of RRL emission that do not appear to be Hiiregions based on their lack of mid-infrared emission. This latter group may be a compact component of the Galactic Diffuse Ionized Gas. There are an additional 10 discrete sources that have anomalously high RRL velocities for their locations in the Galactic plane. We compare these objects’ RRL data to¹³CO, Hi,and mid-infrared data, and find that these sources do not have the expected 24μm emission characteristic of Hiiregions. Based on this comparison we do not think these objects are Hiiregions, but we are unable to classify them as a known type of object. 

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2. The dense warm ionized medium in the inner Galaxy

   https://doi.org/10.1051/0004-6361/202040223  

   Langer, W. D.; Pineda, J. L.; Goldsmith, P. F.; Chambers, E. T.; Riquelme, D.; Anderson, L. D.; Luisi, M.; Justen, M.; Buchbender, C. (July 2021, Astronomy & Astrophysics)

   null (Ed.)

   Context. Ionized interstellar gas is an important component of the interstellar medium and its lifecycle. The recent evidence for a widely distributed highly ionized warm interstellar gas with a density intermediate between the warm ionized medium (WIM) and compact H  II regions suggests that there is a major gap in our understanding of the interstellar gas. Aims. Our goal is to investigate the properties of the dense WIM in the Milky Way using spectrally resolved SOFIA GREAT [N  II ] 205 μm fine-structure lines and Green Bank Telescope hydrogen radio recombination lines (RRL) data, supplemented by spectrally unresolved Herschel PACS [N  II ] 122μm data, and spectrally resolved 12 CO. Methods. We observed eight lines of sight (LOS) in the 20° < l < 30° region in the Galactic plane. We analyzed spectrally resolved lines of [N  II ] at 205 μm and RRL observations, along with the spectrally unresolved Herschel PACS 122 μm emission, using excitation and radiative transfer models to determine the physical parameters of the dense WIM. We derived the kinetic temperature, as well as the thermal and turbulent velocity dispersions from the [N  II ] and RRL linewidths. Results. The regions with [N  II ] 205 μm emission are characterized by electron densities, n ( e ) ~ 10−35 cm −3 , temperatures range from 3400 to 8500 K, and nitrogen column densities N(N + ) ~ 7 × 10 16 to 3 × 10 17 cm −2 . The ionized hydrogen column densities range from 6 × 10 20 to 1.7 × 10 21 cm −2 and the fractional nitrogen ion abundance x (N + ) ~ 1.1 × 10 −4 to 3.0 × 10 −4 , implying an enhanced nitrogen abundance at a distance ~4.3 kpc from the Galactic Center. The [N  II ] 205 μm emission lines coincide with CO emission, although often with an offset in velocity, which suggests that the dense warm ionized gas is located in, or near, star-forming regions, which themselves are associated with molecular gas. Conclusions. These dense ionized regions are found to contribute ≳50% of the observed [C  II ] intensity along these LOS. The kinetic temperatures we derive are too low to explain the presence of N + resulting from electron collisional ionization and/or proton charge transfer of atomic nitrogen. Rather, these regions most likely are ionized by extreme ultraviolet (EUV) radiation from nearby star-forming regions or as a result of EUV leakage through a clumpy and porous interstellar medium. 

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3. The Green Bank Telescope Diffuse Ionized Gas Survey (GDIGS)

   Rao, M.; Wenger, T. V.; Balser, D. S.; Anderson, L. D.; Armentrout, W. P.; Bania, T. M.; Budaiev, N.; Haffner, L. M.; Liu, B.; Luisi, M. (January 2020, American Astronomical Society meeting #235)

   The Warm Ionized Medium (WIM) is a low density, diffuse ionized component of the Milky Way. The WIM is the last major component of the interstellar medium to be studied at high spatial and spectral resolution, and therefore many of its fundamental properties are not clear. Radiation from massive, OB-type stars, which live in the inner galaxy, is thought to escape discrete HII regions to ionize the WIM. However, the inner Galaxy has not been well studied due to extinction from dust at optical wavelengths. GDIGS is a fully-sampled Radio Recombination Line (RRL) survey of the inner Galactic Plane at C-band (4-8 GHz). RRL emission is not affected by extinction from dust, and GDIGS has sufficient spatial resolution to distinguish between HII regions and the WIM emission. Here we discuss the status of GDIGS and some preliminary results from the spectral analysis of the RRLs. 

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4. Discovery of Radio Recombination Lines from Proplyds in the Orion Nebula Cluster

   https://doi.org/10.3847/1538-4357/adbbcc  

   Boyden, Ryan D; Emig, Kimberly L; Ballering, Nicholas P; Law, Charles J; Haworth, Thomas J; Tan, Jonathan C; Cleeves, L Ilsedore; Li, Zhi-Yun (April 2025, The Astrophysical Journal)

   Abstract We present new Atacama Large Millimeter/submillimeter Array observations that, for the first time, detect hydrogen and helium radio recombination lines from a protoplanetary disk. We imaged the Orion Nebula Cluster at 3.1 mm with a spectral setup that covered then= 42 → 41 transitions of hydrogen (H41α) and helium (He41α). The unprecedented sensitivity of these observations enables us to search for radio recombination lines toward the positions of ∼200 protoplanetary disks. We detect H41αfrom 17 disks, all of which are HST-identified “proplyds.” The detected H41αemission is spatially coincident with the locations of proplyd ionization fronts, indicating that proplyd H41αemission is produced by gas that has been photoevaporated off the disk and ionized by UV radiation from massive stars. We measure the fluxes and widths of the detected H41αlines and find line fluxes of ∼30–800 mJy km s⁻¹and line widths of ∼30–90 km s⁻¹. The derived line widths indicate that the broadening of proplyd H41αemission is dominated by outflowing gas motions associated with external photoevaporation. The derived line fluxes, when compared with measurements of 3.1 mm free–free flux, imply that the ionization fronts of H41α-detected proplyds have electron temperatures of ∼6000–11,000 K and electron densities of ∼10⁶–10⁷cm⁻³. Finally, we detect He41αtoward one H41α-detected source and find evidence that this system is helium-rich. Our study demonstrates that radio recombination lines are readily detectable in ionized photoevaporating disks, providing a new way to measure disk properties in clustered star-forming regions. 

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5. The Diffuse Ionized Gas Halo of the Large Magellanic Cloud

   https://doi.org/10.3847/1538-4357/acc06e  

   Smart, B. M.; Haffner, L. M.; Barger, K. A.; Ciampa, D. A.; Hill, A. S.; Krishnarao, D.; Madsen, G. J. (May 2023, The Astrophysical Journal)

   Abstract The Large Magellanic Cloud (LMC) has an extensive Hαemission halo that traces an extended, warm ionized component of its interstellar medium. Using the Wisconsin HαMapper telescope, we present the first kinematic Hαsurvey of an extensive region around the LMC, from (ℓ,b) = (264.°5, − 45.°5) to (295.°5, − 19.°5), covering +150 ≤v_LSR≤ + 390 km s⁻¹. We find that ionized hydrogen exists throughout the galaxy and extends several degrees beyond detected neutral hydrogen emission $(\log \left(N_{\mathrm{H}\mathrm{I}/{\mathrm{cm}}^{-2}}\right)\approx 18.3)$as traced by 21 cm in current surveys. Using the column density structure of the neutral gas and stellar line-of-sight depths as a guide, we estimate the upper limit mass of the ionized component of the LMC to be roughlyM_ionized≈ (0.6–1.8) × 10⁹M_☉, which is comparable to the total neutral atomic gas mass in the same region (M_neutral≈ 0.76–0.85 × 10⁹M_☉). Considering only the atomic phases, we findM_ionized/M_{ionized+neutral}, to be 46%–68% throughout the LMC and its extended halo. Additionally, we find an ionized gas cloud that extends off of the LMC at (ℓ,b) ≈ (285°, − 28°) into a region previously identified as the Leading Arm complex. This gas is moving at a similar line-of-sight velocity as the LMC and hasM_ionized/M_{ionized+neutral}= 13%–51%. This study, combined with previous studies of the SMC and extended structures of the Magellanic Clouds, continues to suggest that warm, ionized gas is as massive and dynamically important as the neutral gas in the Magellanic System. 

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