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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. Revisiting the Vertical Distribution of HI Absorbing Clouds

   Wenger, Trey; Rybarczyk, Daniel; Stanimirovic, Snezana (February 2024, Bulletin American Astronomical Society)

   The three-dimensional distribution of neutral hydrogen in the Milky Way disk is a key constraint on models of Galactic spiral structure, galaxy evolution, and star formation. In particular, the vertical distributions of the different phases of hydrogen (ionized, warm neutral, cold neutral, and molecular) inform our understanding of the evolution of gas between these phases. Although the scale height of the HI emission disk has been well-characterized across the Galaxy, the vertical distribution of the cold HI component is significantly more challenging to constrain due to the sensitive absorption observations required to characterize this phase. Almost four decades ago, Crovisier (1987) pioneered a kinematic method to estimate the vertical distribution of cold HI clouds in the solar neighborhood using the latest results from the Nancay 21-cm absorption survey. This method was subsequently used in other studies to constrain the vertical distribution of neutral and molecular clouds. We have discovered an error in Crovisier's method that can lead to a factor of two inaccuracy in the inferred scale height. We will discuss the mistake and, using the original Nancay data and a corrected method based on Crovisier's technique, demonstrate the magnitude of the error in the inferred scale height of the local cold HI disk. Furthermore, we will introduce a new Monte Carlo Markov Chain method to infer the vertical distribution of HI absorbing clouds with fewer assumptions and better accuracy. This method will be used with the latest HI absorption data from the Galactic ASKAP HI survey of the Milky Way disk to provide an unprecedented view of the 3D distribution of the cold neutral medium in the solar neighborhood. 

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3. 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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4. SHIELD: EVLA HI Spectral Line Observations of Low-mass Dwarfs

   Miazzo, Masao; Ruvolo, Elizabeth; Cannon, John M.; McNichols, Andrew; Teich, Yaron; Adams, Elizabeth A.; Giovanelli, Riccardo; Haynes, Martha P.; McQuinn, Kristen B.; Salzer, John Joseph; et al (January 2017, American Astronomical Society ... meeting)

   The “Survey of HI in Extremely Low-mass Dwarfs” (SHIELD) is a multiwavelength study of local volume low-mass galaxies. Using the now-complete Arecibo Legacy Fast ALFA (ALFALFA) source catalog, 82 systems are identified that meet distance, line width, and HI flux criteria for being gas-rich, low-mass galaxies. These systems harbor neutral gas reservoirs smaller than 3x10^7 M_sun, thus populating the faint end of the HI mass function with statistical confidence for the first time. Here we present new Karl G. Jansky Very Large Array D-configuration HI spectral line observations of 32 previously unobserved galaxies. These low angular resolution (~40" beam) images localize the HI gas; with a few exceptions, the HI gas is co-spatial with the optical centers of the galaxies. These images provide the first glimpse of the neutral interstellar medium in these systems.Support for this work was provided by NSF grant 1211683 to JMC at Macalester College. 

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5. SHIELD: Observations of Three Candidate Interacting Systems

   Ruvolo, Elizabeth; Miazzo, Masao; Cannon, John M.; McNichols, Andrew; Teich, Yaron; Adams, Elizabeth A.; Giovanelli, Riccardo; Haynes, Martha P.; McQuinn, Kristen B.; Salzer, John Joseph; et al (January 2017, American Astronomical Society ... meeting)

   The “Survey of HI in Extremely Low-mass Dwarfs” (SHIELD) is a multiwavelength study of local volume low-mass galaxies. Using the now-complete Arecibo Legacy Fast ALFA (ALFALFA) source catalog, 82 systems are identified that meet distance, line width, and HI flux criteria for being gas-rich, low-mass galaxies. These systems harbor neutral gas reservoirs smaller than 3x10^7 M_sun, thus populating the faint end of the HI mass function with statistical confidence for the first time. In a companion poster, we present new Karl G. Jansky Very Large Array D-configuration HI spectral line observations of 32 previously unobserved galaxies. Three galaxies in that study have been discovered to lie in close angular proximity to more massive galaxies. Here we present VLA HI imaging of these candidate interacting systems. We compare the neutral gas morphology and kinematics with optical images from SDSS. We discuss the frequency of low-mass galaxies undergoing tidal interaction in the complete SHIELD sample.Support for this work was provided by NSF grant 1211683 to JMC at Macalester College. 

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