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We have carried out deep, high velocity resolution, interferometric GalacticHI 21 cm absorption spectroscopy towards 32 compact extra-galactic radiosources with the Giant Metrewave Radio Telescope (GMRT) and the WesterborkSynthesis Radio Telescope (WSRT). The optical depth spectra for most sourceshave root mean square noise values \lesssim 10^{-3} per 1 km/s velocity channeland are thus sufficiently sensitive to detect absorption by warm neutralhydrogen with HI column densities N_HI \gtrsim 10^{20} cm^{-2}, spintemperatures T_s \leq 5000 K, and line widths equal to the thermal width (20km/s). HI 21 cm absorption was detected against all background sources but one,B0438-436. The spectra of sources observed separately with GMRT and WSRT showexcellent agreement, indicating that spectral baseline problems andcontamination from HI 21 cm emission are negligible. This paper presents theabsorption spectra, the emission spectra along neighbouring sightlines from theLeiden-Argentine-Bonn survey, and the derived spin temperature spectra. Onevery sightline, the maximum spin temperature detected (at \ge 3 sigmasignificance) even at a velocity resolution of 1 km/s is \gtrsim 1000 K,indicating that we are detecting the warm neutral medium along most sightlines.This is by far the largest sample of Galactic HI 21 cm absorption spectra ofthis quality, providing a sensitive probe of physical conditions in the neutralatomic ISM.

The temperature of the diffuse HI in the Milky Way I: High resolution HI 21 cm absorption studies