Detection of the Galactic warm neutral medium in H I 21-cm absorption

We report a deep Giant Metrewave Radio Telescope search for Galactic H I 21-cm absorption towards the quasar B0438-436, yielding the detection of wide, weak H I 21-cm absorption, with a velocity-integrated H I 21-cm optical depth of 0.0188 ± 0.0036 km s^-1. Comparing this with the H I column density measured in the Parkes Galactic All-Sky Survey gives a column density-weighted harmonic mean spin temperature of 3760 ± 365 K, one of the highest measured in the Galaxy. This is consistent with most of the H I along the sightline arising in the stable warm neutral medium. The low-peak H I 21-cm optical depth towards B0438-436 implies negligible self-absorption, allowing a multi-Gaussian joint decomposition of the H I 21-cm absorption and emission spectra. This yields a gas kinetic temperature of T_k ≤ (4910 ± < t> 1900) K, and a spin temperature of T_s = (1000 ± 345) K for the gas that gives rise to the H I 21-cm absorption. Our data are consistent with the H I 21-cm absorption arising from either the stable WNM, with T_s ≪ T_k, T_k ≈ 5000 K, and little penetration of the background Lyman-α radiation field into the neutral hydrogen, or the unstable neutral medium, with T_s ≈ T_k ≈ 1000 K.