Abstract
ABSTRACT The 21 cm hyperfine transition of the atomic hydrogen (H i) 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, H i 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 H i 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 H i 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.
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Schedule a callMore From: Monthly Notices of the Royal Astronomical Society: Letters
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