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1. Atomic Hydrogen in the Milky Way: A Stepping Stone in the Evolution of Galaxies

   https://doi.org/10.1146/annurev-astro-052920-104851  

   McClure-Griffiths, Naomi M. ; Stanimirović, Snežana ; Rybarczyk, Daniel R. ( August 2023 , Annual Review of Astronomy and Astrophysics)

   Atomic hydrogen (Hi) is a critical stepping stone in the gas evolution cycle of the interstellar medium (ISM) of the Milky Way. Hi traces both the cold, premolecular state before star formation and the warm, diffuse ISM before and after star formation. This review describes new, sensitive Hi absorption and emission surveys, which, together with high angular and spectral resolution Hi emission data, have revealed the physical properties of Hi, its structure, and its association with magnetic fields. We give an overview of the Hi phases and discuss how Hi properties depend on the environment and what its structure can tell us about feedback in the ISM. Key findings include the following: ▪ The mass fraction of the cold neutral medium is ≲40% on average, increasing with A V due to the increase of mean gas density. ▪ The cold disk extends to at least R ∼ 25 kpc. ▪ Approximately 40% of the Hi is warm, with structural characteristics that derive from feedback events. ▪ Cold Hi is highly filamentary, whereas warm Hi is more smoothly distributed. We summarize future observational and simulation opportunities that can be used to unravel the 3D structure of the atomic ISM and the effects of heating and cooling on Hi properties. 

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2. Examining the Limits of an Artificial Neural Network in Predicting the HI Content of Galaxies

   Rea, David G. ; Rosenberg, Jessica ; Ellison, Sara L. ; Teimoorinia, Hossen ( January 2019 , American Astronomical Society, AAS Meeting)

   The neutral hydrogen (HI) in galaxies provides the gas reservoir out of which stars are formed. The ability to determine the HI masses for statistically significant samples of galaxies can provide information about the connection between this gas reservoir and the star formation that drives galaxy evolution. However, there are relatively few galaxies for which HI masses are known because these measurements are significantly more difficult to make than optical observations. Artificial neural networks are a type of nonlinear technique that have been used estimate the gas masses from their optical properties (Teimoorinia et al. 2017). We present HI observations of 51 galaxies with gas and stellar properties that are rare in the Arecibo Legacy Fast ALFA Survey (ALFALFA, Haynes et al. 2018) which was used to train the Artificial Neural Network developed by Teimoorinia et al. (ANN, 2017). These sources provide a test of the Artificial Neural Network predictions of HI mass and include some rare and interesting systems including galaxies that are extremely massive in both stellar mass (log M_∗> 11.0) and HI mass (log M_HI> 10.2) with large HI line widths (w_50> 500 km/s). We find that this Artificial Neural Network systematically overestimates the gas fraction of the galaxies in our selected sample, suggesting that care must be taken when using these techniques to predict gas masses for galaxies from a broad range of optical properties. 

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3. A ∼15 kpc outflow cone piercing through the halo of the blue compact metal-poor galaxy SBS 0335–052E

   https://doi.org/10.1051/0004-6361/202244930  

   Herenz, E. C. ; Inoue, J. ; Salas, H. ; Koenigs, B. ; Moya-Sierralta, C. ; Cannon, J. M. ; Hayes, M. ; Papaderos, P. ; Östlin, G. ; Bik, A. ; et al ( February 2023 , Astronomy & Astrophysics)

   Context. Outflows from low-mass star-forming galaxies are a fundamental ingredient for models of galaxy evolution and cosmology. Despite seemingly favourable conditions for outflow formation in compact starbursting galaxies, convincing observational evidence for kiloparsec-scale outflows in such systems is scarce. Aims. The onset of kiloparsec-scale ionised filaments in the halo of the metal-poor compact dwarf SBS 0335−052E was previously not linked to an outflow. In this paper we investigate whether these filaments provide evidence for an outflow. Methods. We obtained new VLT/MUSE WFM and deep NRAO/VLA B-configuration 21 cm data of the galaxy. The MUSE data provide morphology, kinematics, and emission line ratios of H β /H α and [O  III ] λ 5007/H α of the low surface-brightness filaments, while the VLA data deliver morphology and kinematics of the neutral gas in and around the system. Both datasets are used in concert for comparisons between the ionised and the neutral phase. Results. We report the prolongation of a lacy filamentary ionised structure up to a projected distance of 16 kpc at SB H α  = 1.5 × 10 −18 erg s cm −2 arcsec −2 . The filaments exhibit unusual low H α /H β  ≈ 2.4 and low [O  III ]/H α  ∼ 0.4 − 0.6 typical of diffuse ionised gas. They are spectrally narrow (∼20 km s −1 ) and exhibit no velocity sub-structure. The filaments extend outwards from the elongated H  I halo. On small scales, the N HI peak is offset from the main star-forming sites. The morphology and kinematics of H  I and H  II reveal how star-formation-driven feedback interacts differently with the ionised and the neutral phase. Conclusions. We reason that the filaments are a large-scale manifestation of star-formation- driven feedback, namely limb-brightened edges of a giant outflow cone that protrudes through the halo of this gas-rich system. A simple toy model of such a conical structure is found to be commensurable with the observations. 

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4. Nitrogen-enriched, Highly Pressurized Nebular Clouds Surrounding a Super Star Cluster at Cosmic Noon

   https://doi.org/10.3847/1538-4357/acf75c  

   Pascale, Massimo ; Dai, Liang ; McKee, Christopher F. ; Tsang, Benny T. -H. ( November 2023 , The Astrophysical Journal)

   Strong lensing offers a precious opportunity for studying the formation and early evolution of super star clusters that are rare in our cosmic backyard. The Sunburst Arc, a lensed Cosmic Noon galaxy, hosts a young super star cluster with escaping Lyman continuum radiation. Analyzing archival Hubble Space Telescope images and emission line data from Very Large Telescope/MUSE and X-shooter, we construct a physical model for the cluster and its surrounding photoionized nebula. We confirm that the cluster is ≲4 Myr old, is extremely massiveM_⋆∼ 10⁷M_⊙, and yet has a central component as compact as several parsecs, and we find a gas-phase metallicityZ= (0.22 ± 0.03)Z_⊙. The cluster is surrounded by ≳10⁵M_⊙of dense clouds that have been pressurized toP∼ 10⁹K cm⁻³by perhaps stellar radiation at within 10 pc. These should have large neutral columnsN_HI> 10^22.8cm⁻²to survive rapid ejection by radiation pressure. The clouds are likely dusty as they show gas-phase depletion of silicon, and may be conducive to secondary star formation ifN_HI> 10²⁴cm⁻²or if they sink farther toward the cluster center. Detecting strong [Niii]λλ1750,1752, we infer heavy nitrogen enrichment$\log (\mathrm{N}/\mathrm{O})=-{0.21}_{-0.11}^{+0.10}$. This requires efficiently retaining ≳500M_⊙of nitrogen in the high-pressure clouds from massive stars heavier than 60M_⊙up to 4 Myr. We suggest a physical origin of the high-pressure clouds from partial or complete condensation of slow massive star ejecta, which may have an important implication for the puzzle of multiple stellar populations in globular clusters.

    

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5. Explanation of Heliospheric Energetic Neutral Atom Fluxes Observed by the Interstellar Boundary Explorer

   https://doi.org/10.3847/2041-8213/ac92e2  

   Zirnstein, E. J. ; Kim, T. K. ; Dayeh, M. A. ; Rankin, J. S. ; McComas, D. J. ; Swaczyna, P. ( September 2022 , The Astrophysical Journal Letters)

   Interstellar neutral atoms propagating into the heliosphere experience charge exchange with the supersonic solar wind (SW) plasma, generating ions that are picked up by the SW. These pickup ions (PUIs) constitute ∼25% of the proton number density by the time they reach the heliospheric termination shock (HTS). Preferential acceleration of PUIs at the HTS leads to a suprathermal, kappa-like PUI distribution in the heliosheath, which may be further heated in the heliosheath by traveling shocks or pressure waves. In this study, we utilize a dynamic, 3D magnetohydrodynamic model of the heliosphere to show that dynamic heating of PUIs at the HTS and in the inner heliosheath (IHS), as well as a background source of energetic neutral atoms (ENAs) from outside the heliopause, can explain the heliospheric ENA signal observed by the Interstellar Boundary Explorer (IBEX) in the Voyager 2 direction. We show that the PUI heating process at the HTS is characterized by a polytropic index larger than 5/3, likely ranging betweenγ∼ 2.3 and 2.7, depending on the time in solar cycle 24 and SW conditions. The ENA fluxes at energies >1.5 keV show large-scale behavior in time with the solar cycle and SW dynamic pressure, whereas ENAs < 1.5 keV primarily exhibit random-like fluctuations associated with SW transients affecting the IHS. We find that ≲20% of the ENAs observed at ∼0.5–6 keV come from other sources, likely from outside the heliopause as secondary ENAs. This study offers the first model replication of the intensity and evolution of IBEX-Hi ENA observations from the outer heliosphere.

    

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