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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. 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 ( December 2023 , Monthly Notices of the Royal Astronomical Society)

   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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3. ALMA multiline survey of the ISM in two quasar host–companion galaxy pairs at z > 6

   https://doi.org/10.1051/0004-6361/202039696  

   Pensabene, A. ; Decarli, R. ; Bañados, E. ; Venemans, B. ; Walter, F. ; Bertoldi, F. ; Fan, X. ; Farina, E. P. ; Li, J. ; Mazzucchelli, C. ; et al ( August 2021 , Astronomy & Astrophysics)

   null (Ed.)

   We present a multiline survey of the interstellar medium (ISM) in two z  > 6 quasar host galaxies, PJ231−20 ( z  = 6.59) and PJ308−21 ( z  = 6.23), and their two companion galaxies. Observations were carried out using the Atacama Large (sub-)Millimeter Array (ALMA). We targeted 11 transitions including atomic fine-structure lines (FSLs) and molecular lines: [NII] 205 μm , [CI] 369 μm , CO ( J up  = 7, 10, 15, 16), H 2 O 3 12  − 2 21 , 3 21  − 3 12 , 3 03  − 2 12 , and the OH 163 μm doublet. The underlying far-infrared (FIR) continuum samples the Rayleigh-Jeans tail of the respective dust emission. By combining this information with our earlier ALMA [CII] 158 μm observations, we explored the effects of star formation and black hole feedback on the ISM of the galaxies using the CLOUDY radiative transfer models. We estimated dust masses, spectral indexes, IR luminosities, and star-formation rates from the FIR continuum. The analysis of the FSLs indicates that the [CII] 158 μm and [CI] 369 μm emission arises predominantly from the neutral medium in photodissociation regions (PDRs). We find that line deficits agree with those of local luminous IR galaxies. The CO spectral line energy distributions (SLEDs) reveal significant high- J CO excitation in both quasar hosts. Our CO SLED modeling of the quasar PJ231−20 shows that PDRs dominate the molecular mass and CO luminosities for J up  ≤ 7, while the J up  ≥ 10 CO emission is likely driven by X-ray dissociation regions produced by the active galactic nucleus (AGN) at the very center of the quasar host. The J up  > 10 lines are undetected in the other galaxies in our study. The H 2 O 3 21  − 3 12 line detection in the same quasar places this object on the L H 2 O  −  L TIR relation found for low- z sources, thus suggesting that this water vapor transition is predominantly excited by IR pumping. Models of the H 2 O SLED and of the H 2 O-to-OH 163 μm ratio point to PDR contributions with high volume and column density ( n H  ∼ 0.8 × 10 5 cm −3 , N H  = 10 24 cm −2 ) in an intense radiation field. Our analysis suggests a less highly excited medium in the companion galaxies. However, the current data do not allow us to definitively rule out an AGN in these sources, as suggested by previous studies of the same objects. This work demonstrates the power of multiline studies of FIR diagnostics in order to dissect the physical conditions in the first massive galaxies emerging from cosmic dawn. 

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4. 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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5. The Role of Neutral Hydrogen in Setting the Abundances of Molecular Species in the Milky Way’s Diffuse Interstellar Medium. II. Comparison between Observations and Theoretical Models

   https://doi.org/10.3847/1538-4357/ac4160  

   Rybarczyk, Daniel R. ; Gong, Munan ; Stanimirović, Snežana ; Babler, Brian ; Murray, Claire E. ; Winters, Jan Martin ; Luo, Gan ; Dame, T. M. ; Steffes, Lucille ( February 2022 , The Astrophysical Journal)

   We compare observations of Hifrom the Very Large Array (VLA) and the Arecibo Observatory and observations of HCO⁺from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA) in the diffuse (A_V≲ 1) interstellar medium (ISM) to predictions from a photodissociation region (PDR) chemical model and multiphase ISM simulations. Using a coarse grid of PDR models, we estimate the density, FUV radiation field, and cosmic-ray ionization rate (CRIR) for each structure identified in HCO⁺and Hiabsorption. These structures fall into two categories. Structures withT_s< 40 K, mostly withN(HCO⁺) ≲ 10¹²cm⁻², are consistent with modest density, FUV radiation field, and CRIR models, typical of the diffuse molecular ISM. Structures with spin temperatureT_s> 40 K, mostly withN(HCO⁺) ≳ 10¹²cm⁻², are consistent with high density, FUV radiation field, and CRIR models, characteristic of environments close to massive star formation. The latter are also found in directions with a significant fraction of thermally unstable Hi. In at least one case, we rule out the PDR model parameters, suggesting that alternative mechanisms (e.g., nonequilibrium processes like turbulent dissipation and/or shocks) are required to explain the observed HCO⁺in this direction. Similarly, while our observations and simulations of the turbulent, multiphase ISM agree that HCO⁺formation occurs along sight lines withN(H I) ≳ 10²¹cm⁻², the simulated data fail to explain HCO⁺column densities ≳ few × 10¹²cm⁻². Because a majority of our sight lines with HCO⁺had such high column densities, this likely indicates that nonequilibrium chemistry is important for these lines of sight.

    

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