The temperature of the diffuse H i in the Milky Way – I. High resolution H i-21 cm absorption studies

Abstract

We have carried out deep, high velocity resolution, interferometric Galactic H I-21 cm absorption spectroscopy towards 32 compact extragalactic radio sources with the Giant Metrewave Radio Telescope (GMRT) and the Westerbork Synthesis Radio Telescope (WSRT). The optical depth spectra for most sources have root-mean-square noise values ≲10^-3 per 1 km s^-1 velocity channel and are thus sufficiently sensitive to detect absorption by warm neutral hydrogen with H I column densities NHI ≳ 1020 cm^-2 , spin temperatures Ts ≤ 5000 K and line widths equal to the thermal width (20 km s^-1 ). H I-21 cm absorption was detected against all background sources but one, B0438−436. The spectra of sources observed separately with GMRT and WSRT show excellent agreement, indicating that spectral baseline problems and contamination from H I-21 cm emission are negligible. This paper presents the absorption spectra, the emission spectra along neighbouring sightlines from the Leiden–Argentine–Bonn survey and the derived spin temperature spectra. On every sightline, the maximum spin temperature detected (at ≥3σ significance) even at a velocity resolution of 1 km s^-1 is ≳1000 K, indicating that we are detecting the warm neutral medium along most sightlines. This is by far the largest sample of Galactic H I-21 cm absorption spectra of this quality, providing a sensitive probe of physical conditions in the neutral atomic interstellar medium.