TY - JOUR
T1 - The 21-SPONGE H i Absorption Line Survey. I. The Temperature of Galactic H i
AU - Murray, Claire E.
AU - Stanimirović, Snežana
AU - Goss, W. M.
AU - Heiles, Carl
AU - Dickey, John M.
AU - Babler, Brian
AU - Kim, Chang Goo
N1 - Publisher Copyright:
© 2018. The American Astronomical Society. All rights reserved.
PY - 2018/10
Y1 - 2018/10
N2 - 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 (σ τ < 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 ∼ 104 K. We conclude that excitation in excess of collisions likely produces significantly higher WNM T s than predicted by steady-state models.
AB - 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 (σ τ < 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 ∼ 104 K. We conclude that excitation in excess of collisions likely produces significantly higher WNM T s than predicted by steady-state models.
KW - ISM: clouds
KW - ISM: structure
KW - radio lines: ISM
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U2 - 10.3847/1538-4365/aad81a
DO - 10.3847/1538-4365/aad81a
M3 - Article
AN - SCOPUS:85055697895
SN - 0067-0049
VL - 238
JO - Astrophysical Journal, Supplement Series
JF - Astrophysical Journal, Supplement Series
IS - 2
M1 - 14
ER -