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1. The Local Group L-Band Survey: The First Measurements of Localized Cold Neutral Medium Properties in the Low-Metallicity Dwarf Galaxy NGC 6822

   Pingel, Nickolas M; Chen, Hongxing; Stanimirović, Snežana; Koch, Eric W; Leroy, Adam K; Rosolowsky, Erik; Kim, Chang-Goo; Dalcanton, Julianne J; Walter, Fabian; Busch, Michael P; et al (July 2024, Astrophysical journal)

   Measuring the properties of the cold neutral medium (CNM) in low-metallicity galaxies provides insight into heating and cooling mechanisms in early Universe-like environments. We report detections of two localized atomic neutral hydrogen (Hi) absorption features in NGC 6822, a low-metallicity (0.2 Z⊙) dwarf galaxy in the Local Group. These are the first unambiguous CNM detections in a low-metallicity dwarf galaxy outside the Magellanic Clouds. The Local Group L-Band Survey (LGLBS) enabled these detections due to its high spatial (15 pc for Hi emission) and spectral (0.4 km s−1) resolution. We introduce LGLBS and describe a custom pipeline to search for Hi absorption at high angular resolution and extract associated Hi emission. A detailed Gaussian decomposition and radiative transfer analysis of the NGC 6822 detections reveals five CNM components, with key properties: a mean spin temperature of 32±6 K, a mean CNM column density of 3.1×1020 cm−2, and CNM mass fractions of 0.33 and 0.12 for the two sightlines. Stacking non-detections does not reveal low-level signals below our median optical depth sensitivity of 0.05. One detection intercepts a star-forming region, with the Hi absorption profile encompassing the CO (2−1) emission, indicating coincident molecular gas and a depression in high-resolution Hi emission. We also analyze a nearby sightline with deep, narrow Hi self-absorption dips, where the background warm neutral medium is attenuated by intervening CNM. The association of CNM, CO, and Hα emissions suggests a close link between the colder, denser Hi phase and star formation in NGC 6822. 

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2. Simulations of the star-forming molecular gas in an interacting M51-like galaxy

   https://doi.org/10.1093/mnras/stz3600  

   Tress, Robin G.; Smith, Rowan J.; Sormani, Mattia C.; Glover, Simon C. O.; Klessen, Ralf S.; Mac Low, Mordecai-Mark; Clark, Paul C. (January 2020, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present here the first of a series of papers aimed at better understanding the evolution and properties of giant molecular clouds (GMCs) in a galactic context. We perform high-resolution, three-dimensional arepo simulations of an interacting galaxy inspired by the well-observed M51 galaxy. Our fiducial simulations include a non-equilibrium, time-dependent, chemical network that follows the evolution of atomic and molecular hydrogen as well as carbon and oxygen self-consistently. Our calculations also treat gas self-gravity and subsequent star formation (described by sink particles), and coupled supernova feedback. In the densest parts of the simulated interstellar medium (ISM), we reach sub-parsec resolution, granting us the ability to resolve individual GMCs and their formation and destruction self-consistently throughout the galaxy. In this initial work, we focus on the general properties of the ISM with a particular focus on the cold star-forming gas. We discuss the role of the interaction with the companion galaxy in generating cold molecular gas and controlling stellar birth. We find that while the interaction drives large-scale gas flows and induces spiral arms in the galaxy, it is of secondary importance in determining gas fractions in the different ISM phases and the overall star formation rate. The behaviour of the gas on small GMC scales instead is mostly controlled by the self-regulating property of the ISM driven by coupled feedback. 

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3. Spatially resolved star formation and fuelling in galaxy interactions

   https://doi.org/10.1093/mnras/staa2952  

   Moreno, Jorge; Torrey, Paul; Ellison, Sara L; Patton, David R; Bottrell, Connor; Bluck, Asa F; Hani, Maan H; Hayward, Christopher C; Bullock, James S; Hopkins, Philip F; et al (March 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We investigate the spatial structure and evolution of star formation and the interstellar medium (ISM) in interacting galaxies. We use an extensive suite of parsec-scale galaxy-merger simulations (stellar mass ratio = 2.5:1), which employs the ‘Feedback In Realistic Environments-2’ model (fire-2). This framework resolves star formation, feedback processes, and the multiphase structure of the ISM. We focus on the galaxy-pair stages of interaction. We find that close encounters substantially augment cool (H i) and cold-dense (H2) gas budgets, elevating the formation of new stars as a result. This enhancement is centrally concentrated for the secondary galaxy, and more radially extended for the primary. This behaviour is weakly dependent on orbital geometry. We also find that galaxies with elevated global star formation rate (SFR) experience intense nuclear SFR enhancement, driven by high levels of either star formation efficiency (SFE) or available cold-dense gas fuel. Galaxies with suppressed global SFR also contain a nuclear cold-dense gas reservoir, but low SFE levels diminish SFR in the central region. Concretely, in the majority of cases, SFR enhancement in the central kiloparsec is fuel-driven (55 per cent for the secondary, 71 per cent for the primary) – while central SFR suppression is efficiency-driven (91 per cent for the secondary, 97 per cent for the primary). Our numerical predictions underscore the need of substantially larger, and/or merger-dedicated, spatially resolved galaxy surveys – capable of examining vast and diverse samples of interacting systems – coupled with multiwavelength campaigns aimed to capture their internal ISM structure. 

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4. LMC+ : Large-scale mapping of [C II] and [O III] in the LMC molecular ridge: I. Dataset and line ratio analyses

   https://doi.org/10.1051/0004-6361/202555833  

   Fischer, C; Madden, S C; Krabbe, A; Polles, F L; Fadda, D; Tarantino, E; Galliano, F; Chen, C-H R; Abel, N; Beck, Á; et al (October 2025, Astronomy & Astrophysics)

   Context. The fundamental process of star formation in galaxies involves the intricate interplay between the fueling of star formation via molecular gas and the feedback from recently formed massive stars that can, in turn, hinder the conversion of gas into stars. This process, by which galaxies evolve, is also closely connected to the intrinsic properties of the interstellar medium (ISM), such as structure, density, pressure, and metallicity. Aims. To study the role that different molecular and atomic phases of the ISM play in star formation, and to characterize their physical conditions, we zoom into our nearest neighboring galaxy, the Large Magellanic Cloud (LMC; 50 kpc), the most convenient laboratory in which to study the effects of the lower metal abundance on the properties of the ISM. The LMC offers a view of the ISM and star formation conditions in a low-metallicity (Z~ 0.5 Z_⊙) environment similar, in that regard, to the epoch of the peak of star formation in the earlier Universe (z~ 1.5). Following up on studies carried out at galactic scales in low-Z galaxies, we present an unprecedentedly detailed analysis of well-known star-forming regions (SFRs) at a spatial resolution of a few parsecs. Methods. We mapped a 610pc× 260pc region in the LMC molecular ridge in [C II]λ158 µm and the [O III]λ88 µm using the FIFI-LS instrument on the SOFIA telescope. We compared the data with the distribution of the CO(2−1) emission from ALMA, the modeled total infrared luminosity, and the Spitzer/MIPS 24 µm continuum and Hα. Results. We present new large maps of [CII] and [OIII] and perform a first comparison with CO(2−1) line and LTIR emission. We also provide a detailed description of the observing strategy with SOFIA/FIFI-LS and the data reduction process. Conclusions. We find that [CII] and [OIII] emission is associated with the SFRs in the molecular ridge, but also extends throughout the mapped region, and is not obviously associated with ongoing star formation. The CO emission is clumpier than the [C II] emission and we find plentiful [C II] present where there is little CO emission, possibly holding important implications for “CO-dark” gas. We find a clear trend of the L_[C II]/L_TIRratio decreasing with increasing L_TIRin the full range. This suggests a strong link between the “[C II]-deficit” and the local physical conditions instead of global properties. 

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5. Where do stars explode in the ISM? -- The distribution of dense gas around massive stars and supernova remnants in M33

   Sarbadhicary, Sumit K; Wagner, Jordan; Koch, Eric W; Mayker-Chen, Ness; Leroy, Adam K; Lahén, Natalia; Rosolowsky, Erik; Neugent, Kathryn F; Kim, Chang-Goo; Chomiuk, Laura; et al (October 2023, Astrophysical journal)

   Star formation in galaxies is regulated by turbulence, outflows, gas heating and cloud dispersal -- processes which depend sensitively on the properties of the interstellar medium (ISM) into which supernovae (SNe) explode. Unfortunately, direct measurements of ISM environments around SNe remain scarce, as SNe are rare and often distant. Here we demonstrate a new approach: mapping the ISM around the massive stars that are soon to explode. This provides a much larger census of explosion sites than possible with only SNe, and allows comparison with sensitive, high-resolution maps of the atomic and molecular gas from the Jansky VLA and ALMA. In the well-resolved Local Group spiral M33, we specifically observe the environments of red supergiants (RSGs, progenitors of Type II SNe), Wolf-Rayet stars (WRs, tracing stars >30 M⊙, and possibly future stripped-envelope SNe), and supernova remnants (SNRs, locations where SNe have exploded). We find that massive stars evolve not only in dense, molecular-dominated gas (with younger stars in denser gas), but also a substantial fraction (∼45\% of WRs; higher for RSGs) evolve in lower-density, atomic-gas-dominated, inter-cloud media. We show that these measurements are consistent with expectations from different stellar-age tracer maps, and can be useful for validating SN feedback models in numerical simulations of galaxies. Along with the discovery of a 20-pc diameter molecular gas cavity around a WR, these findings re-emphasize the importance of pre-SN/correlated-SN feedback evacuating the dense gas around massive stars before explosion, and the need for high-resolution (down to pc-scale) surveys of the multi-phase ISM in nearby galaxies. 

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