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

TL;DR

This paper introduces a new method to estimate the atomic hydrogen column density from 21 cm emission spectra, accounting for optical depth and temperature variations, improving accuracy in complex interstellar conditions.

Contribution

The paper proposes a novel approach to derive unbiased HI column densities from emission spectra considering optical depth and temperature effects.

Findings

The method provides more accurate column density estimates in complex sightlines.

Application to a large sample reveals insights into the spin temperature distribution.

The approach improves upon previous models by accounting for radiative transfer effects.

Abstract

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.