Abstract:
The 21-cm absorption line observed in several QSO systems is believed to originate in neutral hydrogen clouds in the vicinity of the QSOs. The aim of this paper is to show that 21-cm absorption can also originate in a highly ionized emission-line region with electron plasma frequency close to 1420 MHz (21 cm), through the process of parametric decay instability. This absorption process has a line character in principle since it occurs only when a frequency-matching condition is satisfied. The width of the absorption feature results from the width of the electron density distribution function of the emission-line region. The absorption coefficient for the parametric decay process is much larger than that for the spin-flip transition. A part of the radio continuum is therefore depressed because this absorption process occurs over a range of frequencies.
