The temperature of the neutral Interstellar Medium in the Galaxy

Atomic Hydrogen-21 cm transition (HI) is an excellent tracer to study and understand the properties of the atomic gas in the Galaxy. Using the Westerbork Synthesis Radio Telescope (WSRT), we observed 12 quasar sightlines to detect galactic HI in absorption. We achieve an optical depth RMS of ∼ 1-2 × 10^-3, essential to detect the Warm Neutral Medium (WNM). We detect HI absorption in all our sightlines except along 1006+349, for which we set a strict upper limit on the spin temperature as ⟨ T_s ⟩ > 570 K. We find around 50% of our sightlines have ⟨ T_s ⟩ > 500 K, indicating a WNM dominance. Further, we calculate an upper limit of the CNM fraction along our sightlines and find a median CNM fraction of ∼ 0.12. With our observations, we reconfirm the existence of a threshold column density of ∼ 2 × 10^20 cm^-2 to form CNM in the ISM. Using a two-temperature model of the HI disk, we explore the distribution of spin temperature in the Galactic ISM. We find that a simple fixed axisymmetric two-temperature model could not produce either the observed column density or the integral optical depth. This indicates the existence of a more complex distribution of spin temperatures in the Galaxy.