The temperature of the warm neutral medium in the Milky Way

Abstract

We report high-spectral-resolution Australia Telescope Compact Array (ATCA) H I 21-cm observations resulting in the detection of the warm neutral medium (WNM) of the Galaxy in absorption against two extragalactic radio sources, PKS 1814−637 and PKS 0407−658. The two lines of sight were selected on the basis of the simplicity of their absorption profiles and the strength of the background sources; the high velocity resolution of the spectra then enabled us to estimate the kinetic temperatures of the absorbing gas by fitting multiple Gaussians to the absorption profiles. Four separate WNM components were detected towards the two sources, with peak optical depths τmax = (1.0 ± 0.08) × 10^-2 , (1.4 ± 0.2) × 10^-3 , (2.2 ± 0.5) × 10^-3 and (3.4 ± 0.5) × 10^-3 and kinetic temperatures Tk = 3127 ± 300, 3694 ± 1595, 3500 ± 1354 and 2165 ± 608 K, respectively. All four components were thus found to have temperatures in the thermally unstable range 500 < Tk < 5000 K; this suggests that thermal equilibrium has not been reached throughout the WNM.