  The 21 cm hyperfine transition of the atomic hydrogen (HI) in ground state is
a powerful probe of the neutral gas content of the universe. This radio
frequency transition has been used routinely for decades to observe, both in
emission and absorption, HI in the Galactic interstellar medium as well as in
extragalactic sources. In general, however, it is not trivial to derive the
physically relevant parameters like temperature, density or column density from
these observations. Here, we have considered the issue of column density
estimation from the HI 21 cm emission spectrum for sightlines with a
non-negligible optical depth and a mix of gas at different temperatures. The
complicated radiative transfer and a lack of knowledge about the relative
position of gas clouds along the sightline often make it impossible to uniquely
separate the components, and hinders reliable estimation of column densities in
such cases. Based on the observed correlation between the 21 cm brightness
temperature and optical depth, we propose a method to get an unbiased estimate
of the HI column density using only the 21 cm emission spectrum. This formalism
is further used for a large sample to study the spin temperature of the neutral
interstellar medium.
