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The 21 cm hyperfine transition of the atomic hydrogen (HI) in ground state isa powerful probe of the neutral gas content of the universe. This radiofrequency transition has been used routinely for decades to observe, both inemission and absorption, HI in the Galactic interstellar medium as well as inextragalactic sources. In general, however, it is not trivial to derive thephysically relevant parameters like temperature, density or column density fromthese observations. Here, we have considered the issue of column densityestimation from the HI 21 cm emission spectrum for sightlines with anon-negligible optical depth and a mix of gas at different temperatures. Thecomplicated radiative transfer and a lack of knowledge about the relativeposition of gas clouds along the sightline often make it impossible to uniquelyseparate the components, and hinders reliable estimation of column densities insuch cases. Based on the observed correlation between the 21 cm brightnesstemperature and optical depth, we propose a method to get an unbiased estimateof the HI column density using only the 21 cm emission spectrum. This formalismis further used for a large sample to study the spin temperature of the neutralinterstellar medium.

On estimating the atomic hydrogen column density from the H I 21 cm emission spectra