Is Interstellar Hydrogen capable of Maser Action at 21 Centimetres?

Abstract

THREE sets of observations suggest that the density of interstellar neutral hydrogen may be less than the value usually inferred from 21 cm emission line measurements. This possible discrepancy is found in observations of (a) the incoming high velocity neutral hydrogen clouds which, according to Oort¹, deposit material on to the galactic disk at an alarmingly large rate; (b) the ultraviolet spectra of stars in Orion which show an unexpectedly weak interstellar Lyman-α absorption line^2,3; and (c) the X-ray background at ∼ 50 Å which suggests that the opacity of the Galaxy is less than would be estimated from the observed column density of neutral hydrogen⁴.