2011ApJ...737L..33K

We report the discovery of a threshold in the H I column density of Galactic gas clouds below which the formation of the cold phase of H I is inhibited. This threshold is at N_ H I_= 2x10²⁰/cm2; sight lines with lower H I column densities have high spin temperatures (median T_s ∼ 1800 K), indicating low fractions of the cold neutral medium (CNM), while sight lines with N_ H I_ ≥ 2x10²⁰/cm2 have low spin temperatures (median T_s∼ 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 H I column densities. The threshold is similar to the defining column density of a damped Lyα absorber; this indicates a physical difference between damped and sub-damped Lyα systems, with the latter class of absorbers containing predominantly warm gas.