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

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.