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Date and Time of the Query: 2019-06-19 T01:45:39 PDT
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For refcode 2018ApJS..238...14M:
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2018ApJS..238...14M The 21-SPONGE H I Absorption Line Survey. I. The Temperature of Galactic H I Murray, Claire E.; Stanimirovic, Snezana; Goss, W. M.; Heiles, Carl; Dickey, John M.; Babler, Brian; Kim, Chang-Goo 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 (sigma_ tau_ < 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, T_ s_, and column density) of the cold neutral medium (CNM; T_ s_ < 250 K), thermally unstable medium (UNM; 250 K < T_ s_ < 1000 K), and warm neutral medium (WNM; T_ s_ > 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 < T_ s_ < 4000 K. Following spectral modeling, we detect a stacked residual absorption feature corresponding to WNM with T_ s_ ~ 10^4^ K. We conclude that excitation in excess of collisions likely produces significantly higher WNM T_ s_ than predicted by steady-state models. Key words: ISM: clouds, ISM: structure, radio lines: ISM
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