An HI column density threshold for cold gas formation in the Galaxy
Abstract
We report the discovery of a threshold in the HI column density of Galactic gas clouds below which the formation of the cold phase of HI is inhibited. This threshold is at NHI = 2 × 1020 per cm2; sightlines with lower HI column densities have high spin temperatures (median Ts 1800 K), indicating low fractions of the cold neutral medium (CNM), while sightlines with NHI 2 × 1020 per cm2 have low spin temperatures (median Ts 240 K), implying high CNM fractions. The threshold for CNM formation is likely to arise due to inefficient self-shielding against ultraviolet photons at lower HI column densities. The threshold is similar to the defining column density of a damped Lyman-α absorber; this indicates a physical difference between damped and sub-damped Lyman-α systems, with the latter class of absorbers containing predominantly warm gas.