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The 21-SPONGE H I Absorption Line Survey. I. The Temperature of Galactic H I

Citation

Murray, CE and Stanimirovic, S and Goss, WM and Heiles, C and Dickey, JM and Babler, B and Kim, CG, The 21-SPONGE H I Absorption Line Survey. I. The Temperature of Galactic H I, Astrophysical Journal Supplement Series, 238, (2) Article 14. ISSN 0067-0049 (2018) [Refereed Article]


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Copyright Statement

Copyright 2018 The American Astronomical Society

DOI: doi:10.3847/1538-4365/aad81a

Abstract

We present 21 cm Spectral Line Observations of Neutral Gas with the VLA (21-SPONGE), a Karl G. Jansky Very Large Array (VLA) large project (~600 hr) for measuring the physical properties of Galactic neutral hydrogen (H I). 21-SPONGE is distinguished among previous Galactic H I studies as a result of (1) its exceptional optical depth sensitivity (στ < 10-3 per 0.42 km s-1 channel over 57 lines of sight), (2) matching 21 cm emission spectra with the highest possible angular resolution (∼ 4') from the Arecibo Observatory, and (3) detailed comparisons with numerical simulations for assessing observational biases. We autonomously decompose 21 cm spectra and derive the physical properties (i.e., spin temperature, Ts, and column density) of the cold neutral medium (CNM; Ts  < 250 K), thermally unstable medium (UNM; 250 K < Ts < 1000 K), and warm neutral medium (WNM; Ts > 1000 K) simultaneously. Of the total H I mass observed, 50% is detected in both absorption and emission. The CNM makes up the majority of the absorbing gas (56% ± 10%) and 28% of the total H I mass including gas detected only in emission. We find that 20% of the total H I mass is thermally unstable (41% ± 10% of H I detected in absorption), with no significant variation with Galactic latitude. Finally, although the WNM makes up 52% of the total H I mass, we detect little evidence for WNM absorption with 1000 K < Ts < 4000 K. Following spectral modeling, we detect a stacked residual absorption feature corresponding to WNM with Ts ∼ 104 K. We conclude that excitation in excess of collisions likely produces significantly higher WNM T s than predicted by steady-state models.

Item Details

Item Type:Refereed Article
Keywords:clouds, structure, radio lines, radio astronomy
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:133228
Year Published:2018
Web of Science® Times Cited:32
Deposited By:Mathematics and Physics
Deposited On:2019-06-19
Last Modified:2019-08-08
Downloads:10 View Download Statistics

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