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1. Apertif view of the OH megamaser IRAS 10597+5926: OH 18 cm satellite lines in wide-area H I surveys

   https://doi.org/10.1051/0004-6361/202040019  

   Hess, K. M.; Roberts, H.; Dénes, H.; Adebahr, B.; Darling, J.; Adams, E. A.; de Blok, W. J.; Kutkin, A.; Lucero, D. M.; Morganti, R.; et al (March 2021, Astronomy & Astrophysics)

   null (Ed.)

   We present the serendipitous detection of the two main OH maser lines at 1667 and 1665 MHz associated with IRAS 10597+5926 at z ⊙  = 0.19612 in the untargeted Apertif Wide-area Extragalactic imaging Survey (AWES), and the subsequent measurement of the OH 1612 MHz satellite line in the same source. With a total OH luminosity of log( L / L ⊙ ) = 3.90 ± 0.03, IRAS 10597+5926 is the fourth brightest OH megamaser (OHM) known. We measure a lower limit for the 1667/1612 ratio of R 1612  > 45.9, which is the highest limiting ratio measured for the 1612 MHz OH satellite line to date. OH satellite line measurements provide a potentially valuable constraint by which to compare detailed models of OH maser pumping mechanisms. Optical imaging shows that the galaxy is likely a late-stage merger. Based on published infrared and far ultraviolet fluxes, we find that the galaxy is an ultra-luminous infrared galaxy (ULIRG) with log( L TIR / L ⊙ ) = 12.24 that is undergoing a starburst with an estimated star formation rate of 179 ± 40 M ⊙ yr −1 . These host galaxy properties are consistent with the physical conditions responsible for very bright OHM emission. Finally, we provide an update on the predicted number of OH masers that may be found in AWES and estimate the total number of OH masers that will be detected in each of the individual main and satellite OH 18 cm lines. 

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2. First Extragalactic Detection of Thermal Hydroxyl (OH) 18 cm Emission in M31 Reveals Abundant CO-faint Molecular Gas

   https://doi.org/10.3847/1538-4357/ad3af6  

   Busch, Michael P (May 2024, The Astrophysical Journal)

   Abstract The most abundant interstellar molecule, molecular hydrogen (H₂), is practically invisible in cold molecular clouds. Astronomers typically use carbon monoxide (CO) to trace the bulk distribution and mass of H₂in our galaxy and many others. CO observations alone fail to trace a significant component of molecular gas known as “CO-dark” molecular gas, which can be probed with molecules such as OH and CH. We present an extremely sensitive pilot search for the 18 cm hydroxyl (OH) lines in the Andromeda galaxy (M31) with the 100 m Robert C. Byrd Green Bank Telescope. We successfully detected the 1665 and 1667 MHz OH lines in faint emission. The 1665/1667 MHz line ratio displays the characteristic 5:9 ratio predicted under conditions of local thermodynamic equilibrium. To our knowledge, this is the first detection of nonmaser 18 cm OH emission in another galaxy. We compare our OH and Hiobservations with archival CO (1–0) observations. Our OH detection position overlaps with the previously discovered Arp Outer Arm in CO. Our best estimates show that the amount of H₂traced by OH is 100%–140% higher than the amount traced by CO in this sight line. The amount of dark molecular gas implied by dust data supports this conclusion. We conclude that the 18 cm OH lines hold promise as a valuable tool for mapping of the “CO-dark” and “CO-faint” molecular gas phase in nearby galaxies, especially with upcoming multibeam, phased-array feed receivers on radio telescopes, which will allow for drastically improved mapping speeds of faint signals. 

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3. The discovery of a z  = 0.7092 OH megamaser with the MIGHTEE survey

   https://doi.org/10.1093/mnras/stad3821  

   Jarvis, Matt J; Heywood, Ian; Jewell, Sophie M; Deane, Roger P; Klöckner, H -R; Ponomareva, Anastasia A; Maddox, Natasha; Baker, Andrew J; Bianchetti, Alessandro; Hess, Kelley M; et al (March 2024, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present the discovery of the most distant OH megamaser (OHM) to be observed in the main lines, using data from the MeerKAT International Giga-Hertz Tiered Extragalactic Exploration (MIGHTEE) survey. At a newly measured redshift of z = 0.7092, the system has strong emission in both the 1665 MHz (L ≈ 2500 L⊙) and 1667 MHz (L ≈ 4.5 × 104 L⊙) transitions, with both narrow and broad components. We interpret the broad line as a high-velocity-dispersion component of the 1667 MHz transition, with velocity v ∼ 330 km s−1 with respect to the systemic velocity. The host galaxy has a stellar mass of M⋆ = 2.95 × 1010 M⊙ and a star formation rate of SFR = 371 M⊙ yr−1, placing it ∼1.5 dex above the main sequence for star-forming galaxies at this redshift, and can be classified as an ultraluminous infrared galaxy. Alongside the optical imaging data, which exhibit evidence for a tidal tail, this suggests that the OHM arises from a system that is currently undergoing a merger, which is stimulating star formation and providing the necessary conditions for pumping the OH molecule to saturation. The OHM is likely to be lensed, with a magnification factor of ∼2.5, and perhaps more if the maser emitting region is compact and suitably offset relative to the centroid of its host galaxy’s optical light. This discovery demonstrates that spectral line mapping with the new generation of radio interferometers may provide important information on the cosmic merger history of galaxies. 

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4. Faint absorption of the ground state hyperfine-splitting transitions of hydroxyl at 18 cm in the Galactic disk

   https://doi.org/10.1051/0004-6361/202553998  

   Rugel, M R; Beuther, H; Soler, J D; Goldsmith, P; Anderson, L; Hafner, A; Dawson, J R; Wang, Y; Bihr, S; Wiesemeyer, H; et al (August 2025, Astronomy & Astrophysics)

   The interstellar hydride hydroxyl (OH) is a potential tracer of CO-dark molecular gas. We present new high-sensitivity absorption line observations of the four ground state hyperfine-splitting transitions of OH at 18 cm toward four Galactic and extragalactic continuum sources as follow-up to the THOR survey. We compared these to deep observations of the [C II] 158 µm line at 1.9 THz obtained with the upGREAT instrument on SOFIA, observations of the neutral atomic hydrogen (H I) 21 cm line with the VLA, and CO (J= 2–1) lines obtained with the APEX PI230 receiver at the APEX 12 m sub-mm telescope. We trace OH over a large range of molecular hydrogen column densities of 7.9 × 10¹⁹cm^-2to 4.7 × 10²²cm^-2, and derive OH abundances with respect to molecular and total hydrogen column densities ofX_OH,H2=N_OH/N_H2= 1.2_−0.2^+0.3× 10⁻⁷andX_OH,H=N_OH/N_H= 4.8_−0.8^+0.9× 10⁻⁸, respectively. Increased sensitivity and spectral resolution allowed us to detect weak and narrow features with the lowest column density detected atN_OH= 3.7 × 10¹³cm^-2. The increase in sensitivity is a factor of five in direct comparison at the resolution the OH observations in the THOR survey (1.5 km s^-1). We identify only one OH absorption component out of 23 without CO counterpart, yet several with intermediate molecular gas fractions (f_mol≤ 0.8). A potential association of [C II] 158 μm emission with an OH absorption component is seen toward one sightline. Our results confirm that OH absorption traces molecular gas across diffuse and dense environments of the interstellar medium. At the sensitivity limits of the present observations our detection of only one CO-dark molecular gas feature appears to be tracing only the upper end of the distribution of CO-dark OH features found by previous studies. We conclude that if OH absorption was to be used as a CO-dark molecular gas tracer, deeper observations or stronger background targets are necessary to unveil its full potential as a CO-dark molecular gas tracer, and yet it is not an exclusive tracer of CO-dark molecular gas. For OH hyperfine-splitting transitions in the vicinity of photodissociation regions in W43-South, we detect a spectral and spatial offset between the peak of the inversion of the OH 1612 MHz line and the absorption of the OH 1720 MHz line on the one hand, and the absorption of the OH main lines on the other hand, which provides additional constraints on the interpretation of the OH 18 cm line signatures typical of HII regions. 

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5. An early transition to magnetic supercriticality in star formation

   https://doi.org/10.1038/s41586-021-04159-x  

   Ching, T.-C.; Li, D.; Heiles, C.; Li, Z.-Y.; Qian, L.; Yue, Y. L.; Tang, J.; Jiao, S. H. (January 2022, Nature)

   Abstract Magnetic fields have an important role in the evolution of interstellar medium and star formation 1,2 . As the only direct probe of interstellar field strength, credible Zeeman measurements remain sparse owing to the lack of suitable Zeeman probes, particularly for cold, molecular gas 3 . Here we report the detection of a magnetic field of +3.8 ± 0.3 microgauss through the H  I narrow self-absorption (HINSA) 4,5 towards L1544 6,7 —a well-studied prototypical prestellar core in an early transition between starless and protostellar phases 8–10 characterized by a high central number density 11 and a low central temperature 12 . A combined analysis of the Zeeman measurements of quasar H  I absorption, H  I emission, OH emission and HINSA reveals a coherent magnetic field from the atomic cold neutral medium (CNM) to the molecular envelope. The molecular envelope traced by the HINSA is found to be magnetically supercritical, with a field strength comparable to that of the surrounding diffuse, magnetically subcritical CNM despite a large increase in density. The reduction of the magnetic flux relative to the mass, which is necessary for star formation, thus seems to have already happened during the transition from the diffuse CNM to the molecular gas traced by the HINSA. This is earlier than envisioned in the classical picture where magnetically supercritical cores capable of collapsing into stars form out of magnetically subcritical envelopes 13,14 . 

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