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1. The ionization fraction in OMC-2 and OMC-3

   https://doi.org/10.1051/0004-6361/202140670  

   Salas, P.; Rugel, M. R.; Emig, K. L.; Kauffmann, J.; Menten, K. M.; Wyrowski, F.; Tielens, A. G. (September 2021, Astronomy & Astrophysics)

   Context. The electron density ( n e − ) plays an important role in setting the chemistry and physics of the interstellar medium. However, measurements of n e − in neutral clouds have been directly obtained only toward a few lines of sight or they rely on indirect determinations. Aims. We use carbon radio recombination lines and the far-infrared lines of C + to directly measure n e − and the gas temperature in the envelope of the integral shaped filament (ISF) in the Orion A molecular cloud. Methods. We observed the C102 α (6109.901 MHz) and C109 α (5011.420 MHz) carbon radio recombination lines (CRRLs) using the Effelsberg 100 m telescope at ≈2′ resolution toward five positions in OMC-2 and OMC-3. Since the CRRLs have similar line properties, we averaged them to increase the signal-to-noise ratio of the spectra. We compared the intensities of the averaged CRRLs, and the 158 μm-[CII] and [ 13 CII] lines to the predictions of a homogeneous model for the C + /C interface in the envelope of a molecular cloud and from this comparison we determined the electron density, temperature and C + column density of the gas. Results. We detect the CRRLs toward four positions, where their velocity ( v LSR  ≈ 11 km s −1 ) and widths ( σ v  ≈ 1 km s −1 ) confirms that they trace the envelope of the ISF. Toward two positions we detect the CRRLs, and the 158 μm-[CII] and [ 13 CII] lines with a signal-to-noise ratio ≥5, and we find n e −  = 0.65 ± 0.12 cm −3 and 0.95 ± 0.02 cm −3 , which corresponds to a gas density n H  ≈ 5 × 10 3 cm −3 and a thermal pressure of p th  ≈ 4 × 10 5 K cm −3 . We also constrained the ionization fraction in the denser portions of the molecular cloud using the HCN(1–0) and C 2 H(1–0) lines to x (e − ) ≤ 3 × 10 −6 . Conclusions. The derived electron densities and ionization fraction imply that x (e − ) drops by a factor ≥100 between the C + layer and the regions probed by HCN(1–0). This suggests that electron collisional excitation does not play a significant role in setting the excitation of HCN(1–0) toward the region studied, as it is responsible for only ≈10% of the observed emission. 

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2. The Local Group L -band Survey: Probing Cold Atomic Gas in IC 10 with Neutral Hydrogen Absorption

   https://doi.org/10.3847/1538-4357/ae2c58  

   Stelea, Ioana A; Stanimirović, Snežana; Pingel, Nickolas M; Chen, Hongxing; Koch, Eric W; Leroy, Adam K; Rosolowsky, Erik; Kim, Chang-Goo; Bolatto, Alberto D; Dalcanton, Julianne J; et al (January 2026, The Astrophysical Journal)

   Abstract We present the first localized detections of the cold neutral medium (CNM) in IC 10, offering a rare view of dense atomic gas in a low-metallicity (Z/Z_⊙ ∼ 0.27) dwarf galaxy. As a low-metallicity starburst, IC 10’s interstellar medium conditions could reflect small scale physics conditions that mirror those of early galaxies, providing a unique window into the heating and cooling processes that shaped the interstellar medium in early-Universe environments. Leveraging the high angular (<5″ ∼ 15 pc) and spectral (0.4 km s⁻¹) resolution of the Local GroupL-band Survey, we searched for HIabsorption against nine continuum radio sources and detected absorption along three sightlines corresponding to internal radio emission sources within IC 10. Using Gaussian decomposition and radiative transfer, we characterize the CNM, deriving spin temperatures of ∼30–55 K, column densities of (0.6–3.0) × 10²¹cm⁻², cold HIfractions of ∼21%–37%, and line widths of ∼5.6–13.6 km s⁻¹. For each individual detection of HIabsorption, we find corresponding molecular emission from¹²CO (J= 1–0), HCO⁺(J= 1–0), and HCN (J= 1–0) at similar velocities and with comparable line widths, indicating a well-mixed cold atomic and molecular medium. In IC 10, the CNM shows a clear kinematic connection to the high-density ISM, implying a stronger dynamical coupling with molecular gas than in the Milky Way, in line with expectations for low-metallicity environments. At the ∼15 pc scales probed by slightly extended HIIregions in IC 10, unresolved CNM clouds likely contribute to line blending, so the observed broad HIline widths may partly reflect spatial and kinematic averaging. 

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3. Cold Molecular Gas in Merger Remnants. II. The Properties of Dense Molecular Gas

   https://doi.org/10.3847/1538-4365/ac257a  

   Ueda, Junko; Iono, Daisuke; Yun, Min S; Michiyama, Tomonari; Watanabe, Yoshimasa; Snell, Ronald L; Rosa-González, Daniel; Saito, Toshiki; Vega, Olga; Yamashita, Takuji (December 2021, The Astrophysical Journal Supplement Series)

   Abstract We present the 3 mm wavelength spectra of 28 local galaxy merger remnants obtained with the Large Millimeter Telescope. Sixteen molecular lines from 14 different molecular species and isotopologues were identified, and 21 out of 28 sources were detected in one or more molecular lines. On average, the line ratios of the dense gas tracers, such as HCN (1–0) and HCO⁺(1–0), to¹³CO (1–0) are 3–4 times higher in ultra/luminous infrared galaxies (U/LIRGs) than in non-LIRGs in our sample. These high line ratios could be explained by the deficiency of¹³CO and high dense gas fractions suggested by high HCN (1–0)/¹²CO (1–0) ratios. We calculate the IR-to-HCN (1–0) luminosity ratio as a proxy of the dense gas star formation efficiency. There is no correlation between the IR/HCN ratio and the IR luminosity, while the IR/HCN ratio varies from source to source ((1.1–6.5) × 10³L_☉/(K km s⁻¹pc²)). Compared with the control sample, we find that the average IR/HCN ratio of the merger remnants is higher by a factor of 2–3 than those of the early/mid-stage mergers and nonmerging LIRGs, and it is comparable to that of the late-stage mergers. The IR-to-¹²CO (1–0) ratios show a similar trend to the IR/HCN ratios. These results suggest that star formation efficiency is enhanced by the merging process and maintained at high levels even after the final coalescence. The dynamical interactions and mergers could change the star formation mode and continue to impact the star formation properties of the gas in the postmerger phase. 

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4. A 2–3 mm high-resolution molecular line survey towards the centre of the nearby spiral galaxy NGC 6946

   https://doi.org/10.1051/0004-6361/202142624  

   Eibensteiner, C.; Barnes, A. T.; Bigiel, F.; Schinnerer, E.; Liu, D.; Meier, D. S.; Usero, A.; Leroy, A. K.; Rosolowsky, E.; Puschnig, J.; et al (March 2022, Astronomy & Astrophysics)

   The complex physical, kinematic, and chemical properties of galaxy centres make them interesting environments to examine with molecular line emission. We present new 2 − 4″ (∼75 − 150 pc at 7.7 Mpc) observations at 2 and 3 mm covering the central 50″ (∼1.9 kpc) of the nearby double-barred spiral galaxy NGC 6946 obtained with the IRAM Plateau de Bure Interferometer. We detect spectral lines from ten molecules: CO, HCN, HCO + , HNC, CS, HC 3 N, N 2 H + , C 2 H, CH 3 OH, and H 2 CO. We complemented these with published 1 mm CO observations and 33 GHz continuum observations to explore the star formation rate surface density Σ SFR on 150 pc scales. In this paper, we analyse regions associated with the inner bar of NGC 6946 – the nuclear region (NUC), the northern (NBE), and southern inner bar end (SBE) and we focus on short-spacing corrected bulk (CO) and dense gas tracers (HCN, HCO + , and HNC). We find that HCO + correlates best with Σ SFR , but the dense gas fraction ( f dense ) and star formation efficiency of the dense gas (SFE dense ) fits show different behaviours than expected from large-scale disc observations. The SBE has a higher Σ SFR , f dense , and shocked gas fraction than the NBE. We examine line ratio diagnostics and find a higher CO(2−1)/CO(1−0) ratio towards NBE than for the NUC. Moreover, comparison with existing extragalactic datasets suggests that using the HCN/HNC ratio to probe kinetic temperatures is not suitable on kiloparsec and sub-kiloparsec scales in extragalactic regions. Lastly, our study shows that the HCO + /HCN ratio might not be a unique indicator to diagnose AGN activity in galaxies. 

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5. Close-up view of a luminous star-forming galaxy at z = 2.95

   https://doi.org/10.1051/0004-6361/202039743  

   Berta, S.; Young, A. J.; Cox, P.; Neri, R.; Jones, B. M.; Baker, A. J.; Omont, A.; Dunne, L.; Carnero Rosell, A.; Marchetti, L.; et al (February 2021, Astronomy & Astrophysics)

   Exploiting the sensitivity of the IRAM NOrthern Extended Millimeter Array (NOEMA) and its ability to process large instantaneous bandwidths, we have studied the morphology and other properties of the molecular gas and dust in the star forming galaxy, H-ATLAS J131611.5+281219 (HerBS-89a), at z = 2.95. High angular resolution (0 . ″3) images reveal a partial 1 . ″0 diameter Einstein ring in the dust continuum emission and the molecular emission lines of 12 CO(9−8) and H 2 O(2 02  − 1 11 ). Together with lower angular resolution (0 . ″6) images, we report the detection of a series of molecular lines including the three fundamental transitions of the molecular ion OH + , namely (1 1  − 0 1 ), (1 2  − 0 1 ), and (1 0  − 0 1 ), seen in absorption; the molecular ion CH + (1 − 0) seen in absorption, and tentatively in emission; two transitions of amidogen (NH 2 ), namely (2 02  − 1 11 ) and (2 20  − 2 11 ) seen in emission; and HCN(11 − 10) and/or NH(1 2  − 0 1 ) seen in absorption. The NOEMA data are complemented with Very Large Array data tracing the 12 CO(1 − 0) emission line, which provides a measurement of the total mass of molecular gas and an anchor for a CO excitation analysis. In addition, we present Hubble Space Telescope imaging that reveals the foreground lensing galaxy in the near-infrared (1.15  μ m). Together with photometric data from the Gran Telescopio Canarias, we derive a photometric redshift of z phot = 0.9 −0.5 +0.3 for the foreground lensing galaxy. Modeling the lensing of HerBS-89a, we reconstruct the dust continuum (magnified by a factor μ  ≃ 5.0) and molecular emission lines (magnified by μ  ∼ 4 − 5) in the source plane, which probe scales of ∼0 . ″1 (or 800 pc). The 12 CO(9 − 8) and H 2 O(2 02  − 1 11 ) emission lines have comparable spatial and kinematic distributions; the source-plane reconstructions do not clearly distinguish between a one-component and a two-component scenario, but the latter, which reveals two compact rotating components with sizes of ≈1 kpc that are likely merging, more naturally accounts for the broad line widths observed in HerBS-89a. In the core of HerBS-89a, very dense gas with n H 2  ∼ 10 7 − 9 cm −3 is revealed by the NH 2 emission lines and the possible HCN(11 − 10) absorption line. HerBS-89a is a powerful star forming galaxy with a molecular gas mass of M mol  = (2.1 ± 0.4) × 10 11   M ⊙ , an infrared luminosity of L IR  = (4.6 ± 0.4) × 10 12   L ⊙ , and a dust mass of M dust  = (2.6 ± 0.2) × 10 9   M ⊙ , yielding a dust-to-gas ratio δ GDR  ≈ 80. We derive a star formation rate SFR = 614 ± 59  M ⊙ yr −1 and a depletion timescale τ depl  = (3.4 ± 1.0) × 10 8 years. The OH + and CH + absorption lines, which trace low (∼100 cm −3 ) density molecular gas, all have their main velocity component red-shifted by Δ V  ∼ 100 km s −1 relative to the global CO reservoir. We argue that these absorption lines trace a rare example of gas inflow toward the center of a galaxy, indicating that HerBS-89a is accreting gas from its surroundings. 

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