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

Saha, Preetha and Roy, Nirupam and Bhattacharya, Mukul (2018) On estimating the atomic hydrogen column density from the HI 21 cm emission spectra. In: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 480 (1). L126-L130.

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Official URL: https://doi.org/10.1093/mnrasl/sly139


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 cmbrightness 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.

Item Type: Journal Article
Additional Information: Copyright of this article belongs to OXFORD UNIV PRESS
Keywords: ISM: atoms; ISM: clouds; ISM: general; radio lines: ISM
Department/Centre: Division of Physical & Mathematical Sciences > Physics
Depositing User: Francis Jayakanth
Date Deposited: 07 Feb 2019 08:51
Last Modified: 07 Feb 2019 08:51
URI: http://eprints.iisc.ac.in/id/eprint/61662

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