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The mach HI absorption survey I physical conditions of cold atomic gas outside of the galactic plane


Murray, CE and Stanimirovic, S and Heiles, C and Dickey, JM and McClure-Griffiths, NM and Lee, MY and Goss, WM and Killerby-Smith, N, The mach HI absorption survey I physical conditions of cold atomic gas outside of the galactic plane, Astrophysical Journal, Supplement Series, 256, (2) pp. 1-24. ISSN 0067-0049 (2021) [Refereed Article]

Copyright Statement

2021. The American Astronomical Society. All rights reserved.

DOI: doi:10.3847/1538-4365/ac0f0b


Tracing the transition between the diffuse atomic interstellar medium and cold, dense gas is crucial for deciphering the star formation cycle in galaxies. Here we present Measuring Absorption by Cold Hydrogen (MACH), a new survey of cold neutral hydrogen (HI) absorption at 21 cm by the Karl G. Jansky Very Large Array. We target 42 bright background sources with 60 < l < 110, 30 < b < 62, significantly expanding the sample of publicly available, sensitive 21 cm absorption outside the Galactic plane. With matching 21 cm emission data from the Effelsberg-Bonn Hi (EBHiS) Survey, we measure the total column density and cold Hi fraction, and quantify the properties of individual Hi structures along each sightline via autonomous Gaussian decomposition. Combining the MACH sample with results from recent Hi absorption surveys, we produce a robust characterization of the cool atomic medium at high and intermediate Galactic latitudes. We find that MACH Hi has significantly smaller column density relative to samples at similar latitudes, and the detected cold Hi structures have smaller line widths, temperatures, and turbulent Mach numbers, suggesting that MACH probes a particularly quiescent region. Using all available observations, we compute the cumulative covering fraction (c) of cold Hi at local velocities outside the disk: structures with τ > 0.001 are ubiquitous (c ∼ 100%), whereas high optical depths (τ > 1) are extremely rare (c ∼ 0%).

Item Details

Item Type:Refereed Article
Keywords:neutral hydrogen, dark gas, interstellar-medium, star-formation, 21-sponge HI, line survey, dust, temperature, clouds, sky
Research Division:Physical Sciences
Research Group:Astronomical sciences
Research Field:Galactic astronomy
Objective Division:Expanding Knowledge
Objective Group:Expanding knowledge
Objective Field:Expanding knowledge in the physical sciences
UTAS Author:Dickey, JM (Professor John Dickey)
ID Code:152818
Year Published:2021
Web of Science® Times Cited:2
Deposited By:Mathematics
Deposited On:2022-08-24
Last Modified:2022-09-13

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