Analysis of the Absorption Line Profile at 21 cm for the Hydrogen Atom in Interstellar Medium

TL;DR

This paper investigates how the 21 cm absorption line profile of hydrogen in the interstellar medium is affected by hyperfine and Ly$_\alpha$ transitions, introducing corrections to optical depth calculations for better hydrogen column density estimates.

Contribution

It presents a new theoretical model accounting for Ly$_\alpha$ effects on the 21 cm line profile, providing significant corrections to optical depth measurements.

Findings

The Ly$_\alpha$ transition can substantially modify the 21 cm line profile.

A correction to optical depth due to Ly$_\alpha$ effects is proposed.

Effects of non-Doppler broadening and frequency shifts are analyzed.

Abstract

The paper analyzes the absorption line profile at 21 cm for the hydrogen atom in the interstellar medium. The hydrogen atom is treated as a three-level system illuminated by a powerful light source at neighboring resonances corresponding to the hyperfine splitting of the ground state and Ly$_\alpha$ transition. The field acting upon the resonances gives rise to physical processes, which can be explained as interfering pathways between different transitions. The paper considers particular cases when the 21 cm line profile is substantially modified by the Ly$_\alpha$ transition. A correction to the optical depth is introduced as a result of theory. It is shown that the correction can be considerable and should be taken into account when determining the column density of hydrogen atoms in the interstellar medium. The paper also deals with the effects of none-Doppler broadening and frequency shift.