The 21-SPONGE HI Absorption Line Survey II: The temperature of Galactic HI

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 hours) for measuring the physical properties of Galactic neutral hydrogen (HI). 21-SPONGE is distinguished among previous Galactic HI studies as a result of: (1) exceptional optical depth sensitivity ($\sigma_{\tau} < 10^{-3}$ per $0.42\rm\,km\,s^{-1}$ channels over 57 lines of sight); (2) matching 21 cm emission spectra with highest-possible angular resolution (~4') from the Arecibo Observatory; (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, $T_s$, column density) of the cold neutral medium (CNM; $T_s<250\rm\,K$), thermally unstable medium (UNM; $250< T_s < 1000\rm\,K$) and warm neutral medium (WNM; $T_s > 1000\rm\,K$) simultaneously. We detect 50% of the total HI mass in absorption, the majority of which is CNM (56 +/- 10%, corresponding to 28% of the total HI mass). Although CNM is detected ubiquitously, the CNM fraction along most lines of sight is <50%. We find that 20% of the total HI mass is thermally unstable (41 +/- 10% of HI detected in absorption), with no significant variation with Galactic environment. Finally, although the WNM comprises 52% of the total HI mass, we detect little evidence for WNM absorption with $1000<T_s<4000\rm\,K$. Following spectral modeling, we detect a stacked residual absorption feature corresponding to WNM with $T_s\sim10^4\rm\,K$. We conclude that excitation in excess of collisions likely produces significantly higher WNM $T_s$ than predicted by steady-state models.

Insights into the 21-SPONGE H I Absorption Line Survey II: Temperature of Galactic H I

The presented paper conducts an extensive investigation into the properties of Galactic neutral hydrogen (H I) through the 21-SPONGE survey, employing high-sensitivity observations of the 21-cm line using the Very Large Array (VLA). The study represents a significant advancement in the understanding of the temperature distribution and thermodynamic state of H I across various phases in the interstellar medium (ISM).

High-Sensitivity Observations

This large-scale survey capitalizes on the VLA’s capabilities, successfully integrating both absorption and emission spectra. Through autonomous Gaussian decomposition, the observational strategy offers unprecedented sensitivity to the optical depth of H I, allowing for accurate spin temperature and column density determination across different ISM phases: the cold neutral medium (CNM), unstable neutral medium (UNM), and warm neutral medium (WNM).

Analysis and Findings

The survey reveals several critical insights:

1. CNM Ubiquity: The CNM is detected with remarkable regularity across the examined sightlines, accounting for 56% of the total H I mass detected in absorption and 28% of the entire H I mass, affirming its pervasive presence in the Galaxy. Despite this ubiquity, the CNM fraction along most lines of sight remains below 50%.

2. Thermally Unstable Gas: The survey quantifies the thermally unstable H I mass fraction at 41% among absorption-detected gas, and 20% of total H I mass—a substantial fraction that aligns with prior indirect estimates. This highlights the significant role of turbulence and other non-equilibrium processes in shaping the neutral ISM.

3. WNM Properties: Contrary to standard models predicting WNM spin temperatures between 1000-4000 K, the analysis suggests higher temperatures, potentially exceeding 7200 K, implying additional excitation mechanisms beyond collisional processes. This finding is consistent with models incorporating resonant Lyα scattering (Wouthuysen-Field effect), emphasizing the necessity for advanced excitation frameworks in future simulations.

4. Optical Depth Influences: The study confirms a minimal impact of optically thick H I on column density calculations, undermining claims of large corrections to total gas mass estimates due to optically thick neutral gas.

Future Implications and Theoretical Speculation

The comprehensive set of observational data serves as a benchmark for future surveys, especially those utilizing the next-generation facilities, which may not match 21-SPONGE's optical depth sensitivity but will benefit from its findings. The existing discrepancies between modeled and observed spin temperatures, particularly for the WNM, suggest directions for enhancing theoretical models that account for non-steady-state dynamics and the impact of varying cosmic ray influx or metallicity.

Moreover, the methodology for automated spectral decomposition showcased in this study is poised to facilitate the analysis of incoming data volumes, evolving into a standard tool as astronomical datasets continue to expand.

Conclusion

The 21-SPONGE survey makes significant strides in refining our understanding of the thermal properties and phases of Galactic neutral hydrogen. With its robust methodology and unmatched sensitivity, it lays a foundational framework for ongoing and future explorations into the complexities of the ISM, enhancing both observational and theoretical spheres of astrophysical research.