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

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 (similar to 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 (similar to 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; 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 similar to 10(4) K. We conclude that excitation in excess of collisions likely produces significantly higher WNM T-s than predicted by steady-state models.